You are going to get a lot of great advice here and some may sound harsh/extreme but most of us have been living in this type of step hell situation for a long time.  I would like to tell you it gets better but with a HCGUBM (high conflict golden uteris mom) that is actively engaging in PAS (parental alienation syndrome) you are going to have a lot of issues. 

You are absolutely correct that you should not leave so they will visit.  That is crazy.  Everything their BM is doing to them is and they will likely never bond with you and always resent you as a result of her mind games.

Honestly, you are young and childless.  I know this will sound bad and I don't usually go around saying don't get married but in your case I wouldn't marry this man.  Just wait and see how ugly it gets if you dare decide to have a child with your fiance.  Your child will always be expected to play second fiddle to the sacred "first family"....which I prefer to call the FAILED FAMILY! 

If it were me i would be nice and friendly, but matter-of-fact when saying (only if asked) "This is my home, too. I live here. I have nowhere else to go, and i am staying. I look forward to meeting you when you decide to visit!"

Also, though, like the above poster said - are you....sure? Sure you want to sign up for this when you are young and child-free? Seriously, make sure your future husband has really good boundaries with his ex, treats his kids like kids, and you like the wife. Please meet the kids and see how he interacts with them and you together. Look out for mini-wife syndrome and/or ignoring you, violating your rights, or any other such nonsense when they finally do come over. You are at risk of bait-and switch. You need to know what you are getting into in case BM decides to dump them all on your hisband full-time. 

they are children and don't get a say whatsoever and their daddy can tell them that in whatever nice fluffy way he wants. 

 I would not entertain speaking about any of that with CHILDREN.

Also, you are definatley in for a looong road. As in love as I was and AM with my DH, I would not do it again. These people (HCGUBM) are life,fun,time suckers.

You are too young and smart it sounds like to get caught up in this world. 

If you go forward you and DH will have to put each other 1st always. Good Luck!

Does your fiance have a good custody visitation plan sanctioned by family court? If so, BM cannot dictate when children visit and certainly children cannot dicate visitation.

Thats the first thing you both need to look into. I repeat - children do not dictate visitation schedule.

Secondly, the parental alienation is big with this one. You cannot battle it, not legally really.

So, before you get married, you need to get this whole visitation thing cleared up and visit with the kiddos. See how your fiance interacts with them.

Absolutely you do NOT leave the house and do not even entertaine the idea of such a thing.

Good luck!

I would also think twice about marrying into this disaster. It seems like there are not any healthy boundaries present. If you want your own kiddos, a man with 3 already is not as likely to want more to take on...

I can relate to some of your story (same age gap, same number of SKIDS who were about the same age), but honestly, your situation is quite a bit worse than the one I get myself into (we didn't meet until well after DH's divorce, and HCBM has been very difficult with DH, but has left me alone).  

The thing about leaving your house on the weekends? That is ABSURD!  Like, don't even give it another thought.  Your BF needs to grow some balls and stand up to his exwife and assert his rights as a father to see his kids on his own terms.  He should have shut this down without news of it ever even getting to you.  

I'll tell you the harsh truth based on my experience, but who knows, maybe you're the exception and things will go better for you.  First, it never gets better.  You may be telling yourself that as each kid turns 18 and child support ends, your finances will improve.  NOPE!  In our case, with college and guilty dad syndrome, it actually got worse instead of better as the oldest two aged out of child support.  Second, you are in an impossible situation.  From the description you provided, there is nothing you can do to make the SKIDS like you, and you are wasting the best years of your life trying.  The nicer you are to the SKIDS, the more they will resent you just for existing.  Third, you are 30 and child free!  Do you want to have kids of your own? If yes, then leave as soon as possible.  Having kids with someone who already has 3 kids is no fun.  It's not just that you don't get to share firsts together.  You'll have limited resources available for your own kids because you're still supporting your husband's first family.  Your children will not have a full time dad, because their dad's attention will be divided with his first family, and he'll probably even favor them because of his guilt.  WAY before you're ready, he'll be a grandpa and you'll be a stepgrandma.  Say that out loud to yourself. How does it sound?  Given that you don't even have your own child yet, you shouldn't be thinking "grandparenting" for another 25-30 years.  How about when you go house shopping with your husband for your next house and he brings up having a good bunkroom in the house, because in his mind, he's dreaming about all those times his adult children and their kids will come stay with you.  Won't that be fun when you're 40 years old and playing grandma.   I mean, is this really what you want?  Sorry, I know I'm being harsh but there are some parts of your story that hit close to home and this is what I'd tell younger self.  

How long have you been together? It's a big red flag that he hasn't had the kids meet you.

My best advice is you plan to stay with this guy....is to stay out of it. Act like they don't exist but be cordial when they are there.....if they don't want to see their dad that's on them and BM....I would not assist in facilitating that.

it won't matter if you kiss their was or ignore them....the outcome will be the same.....a long road of stress, drama and resentment.

I did not read the other comments but what your FUTURE HUSBAND needs to tell his kids is very simple:

"Children, you are not in control of my or my fiancee's lives. She will not leave HER home. You will not demand of her, talk down to her or disrespect her. She is in no way at fault for your mother's and my divorce. That was between us. You don't get a say in coming over- we are going to follow a court order. There are rules and expectations in our home and you are going to follow them. None of this is up for discussion. I love you."

Now for the rest of it- you are in for a roller coaster my dear. I am not saying that you shouldn't marry this man because it is obvious why he isn't with BM anymore. He is probably a good and decent man. You just need to decide together how much you are going to let these kids control your lives. They aren't toddlers they are teens and preteens which make them the worst (saying that as a BM of that age group myself). They are hormonal, opinionated, attitude filled jerks some times- that I love with all of my heart and soul but still... jerks. Being a stepmom to other teenage jerks is a lot! There is no bond like I have with my own. My fuse is much shorter. I have to remind myself to breathe... all the time. 

"They want to ask me why I will not leave my house on the weekends and tell me why they are upset with me for marrying their father."

Seriously?????  I am assuming, since you haven't even met these kids, that this message is being delivered to you through your fiance.  Is he nuts?  Why hasn't he nipped this in the bud and told his kids (1) that there is no way you are leaving on weekends because HE doesn't want you to and wouldn't allow it and (2) that who he marries is none of their concern and it would be rude and unacceptable for them to tell you why they don't like him marrying you?

I would pump the brakes on this marriage.  There is no way I would marry a man with children - especially minor children - I hadn't met.  And there is no way I would marry a man whose children already had a problem with me unless he 100% had my back and was prepared to tell his kids to pound sand if they don't like you.  The fact that he is even entertaining the idea of a meeting (virtual or otherwise) where his kids plan to be rude and make you feel uncomfortable (because really, what else is going to happen in that conversation if the stated purpose is as you describe?) is unacceptable.  This is something HE needs to address with his children, and putting you in the line of their fire is not what a concerned and considerate partner would do.

Quite frankly, it doesn't sound like he is ready for another marriage, as he still hasn't cleaned up the mess from his first marriage.

Take this time during your engagement to really watch and listen how your future husband deals with this.

IF he does not re-direct his kids, possibly putting bm in her place TOO if she is the root of this. My guess she is, and tell them to be respectful to you----run.

They do not have to be buddy buddy with you, they don't even have to loveeee you but they do have to be kind, polite and respectful. And you should do the same.

JMO

At the very least, delay the wedding and DO NOT MARRY until you have seen how he interacts with the children around you. Plenty of StepTalkers have relationships that they mistakenly call "perfect" except when the kids visit and they get shafted as their perfect partner morphs into a doormat. Do not dive into marriage with that huge questionmark untested.

I come from a similar situation to yours only a few more years into the relationship. We now have a bio child. The ex is still playing HCGUBM to the alienated now adult SD. I wish I had the same advice you did rather than the dream of "love will carry us through". His children will always be a financial drain. Don't be fooled into counting down the years until they reach 18. There's college expenses, cars,  whatever young adults expect parents to help out with these days, etc. Then there's estate battles. My will has to be carefully worded so that our daughter inherits the bulk of my estate rather than DH. Because if I didn't, my money would become his money and SD is entitled to a portion of his, leaving my daughter shortchanged. Always need to be on guard with every financial investment who's name assets are in. 

On my more depressed days I wish I started a family with someone childless to begin with. Someone devoted to just me and our child. Once I told DH it felt like he married me to keep his life nice and comfy. He gets companionship and someone to pitch into the cost of living into the twilight years of his life. His ex will sit back and get a steady mealticket while making a constant dent in our budget. My youth wasted away supporting someone who will be elderly/dead by the time I reach middle aged. All because in the first moments of our relationship I was blinded by how good it felt to be with him. Nice dates, holidays, funny conversations, the nicest guy to spend time with. I was absolutely smitten. Too in love to consider distant future implications. In too deep to contemplate leaving the relationship for someone better for me. 
 

Your SO has already given control to these kids but allowing them to refuse visitation. He needs to be a parent. I would not want to live with all this hostility. I wouldn't allow them to control that meeting at all. "our relationship is not up for debate. Does your father get to veto your relationships ?"

I didn't read the other responses, so excuse me if I repeat, as I'm sure other wise ones have given the advice I'm going to give.

They have recently requested that they want to start coming again on weekend but only if I leave every other weekend and live somewhere else.

Absolutely not, not even having the discussion. This request should be ignored.

The children have requested to chat with me, via video, but we have said we can chat face to face. They want to ask me why I will not leave my house on the weekends and tell me why they are upset with me for marrying their father.

Nope. Again, the request for you to leave your home should be ignored, at least, by you. Your fiance should tell them that it's rude for them to ask that, that it is your home and you are not leaving it on weekends.

They want to tell you why they are upset that you are marrying their father? I think everybody already knows why they are upset, and you do not have to defend your postition. The minute you start trying, you give them power.

Now, I don't believe that kids should just be forced to accept whatever, without some discussion (they do have feelings), but the discussion should be with their father. HE is the one who needs to address their concerns. If they just want to gang up on you, then no, that should not happen. These types of "chats" rarely turn out well for the "outsider" (you).

Parents need to guide their kids through divorce and the new relationships that happen after the divorce. It is not your job. You can reassure them through your actions when you are with them but these meetings are designed to back you into a corner and give them power and make you uncomfortable. No.

This is going to be a rough road. I am so sorry you are going through this.  The truth is - it's going to get worse before (if ever) it gets less horrible.  The kids have been poisoned to hate you by their mother - this wont change, even if they dont believe the lies she feeds them.

I also was in my 30s and my husband in his 40s with his 3 kids and a high conflict BM.  Do not rush this wedding.  This is going to be so much harder than you can even imagine.  More straight talk - I'm sure you love him.  But you have to love your life too.  There are so many years of hell ahead of you with this situation.