Oy... 3 1/2 months and living together... and you haven't met the kids? BM is going to have a field day with this one. 

If you moved in they are going to notice. You might as well rip the band-aid off and make sure that you guys have a fantastic weekend planned with them the next time he has them. Honestly... maybe instead of going right into the house that you share you can do a weekend away together... camping or an amusement park, or visiting a fun city. That way it isn't right in their faces but you get to meet on really fun and memorable terms? 

The time to meet was before you moved in. 

Justmakingthebest has an excellent idea. I would plan a weekend getaway.  

Personally, I met my SS after about 4 months of dating his father. At the time I met him, he was 3.5 y/o. The relationship between SS and I started off a little rocky. He often glared at me when I was near his father and would not speak to me. He would be playful and laughing with his father, and then immediately stop and stare if I walked into the room. He wanted nothing to do with me. I took this personally at first - but then I realized that he probably didn't like seeing someone else hugging and kissing his dad. This lasted for about 5 months.

Now, SS will be 6 y/o in a few months. He no longer glares at me. Instead, he seems happy to see me and often gives me hugs before he leaves. He even tells his dad that he wants to hold my hand when we are out. It isn't perfect. He still does not listen to me when I ask him to stop doing something or to pick up his toys...but compared to how it was in the beginning, this is an improvement.

As for your question about whether you and your SO should sleep in separate beds - that depends on the child. With me and my SO, we did not live together when I first met his son. I made it a point to not be there before SS woke up, or after SS went to bed. That way he could see me walk into the house, and walk-out at the end of the day. Once we were together for over a year, then I started to stay overnight when he was there, and I slept in the same bed as my SO. I think this gradual change allowed SS to see that I respected his boundaries and that I wanted to make it as comfortable for him as I could. 

I feel like since my SS was on the younger side when he met me, that made the transition a little easier. He didn't grow up seeing his mom and dad together often. Since your Skids are on the older side, I would suggest giving them a little bit of time to adjust. They probably remember seeing their dad and BM together and they may not like the idea that he is with another woman immediately. However, I do think you should sleep in the same bed, especially since you two live together. They are old enough to understand that this is your house with their father. You should not change the way you both live in that house just for them.

If they do not warm up to you right away, don't feel discouraged. It takes time, and is a process. I am still learning, as well.

Bringing in a new partner is never an easy time. I fear that in your 'surprise' instant new SM tactic is going to not go well. 

You've never met the children. They didn't even know you existed. But suddenly there is this new strange lady living in Daddy's house? *SMDH*

You're looking at three little kids , all who have different personalities , who are going to walk into your front door. They don't know you from the man on the moon. You don't know them. Except for perhaps what your BF has told you, you have no idea how he is with his children. What kind of parent he is. What his home with the kids in it is like during their visits. 

I fear you've set both the kids and yourself up for failure right off the bat.Sure, you ave met and fall in love with BF, but that was a time of nothing but love and romance and plenty of one on one attention and no daily real life .... kind of like a honeymoon. 

Now here comes reality. 

I wish you well. You have no idea what you just signed up for. 

How would you put off meeting them?  Do you have another place to go?  Does your SO visit with them outside of the house?  If not, I don't think you have much choice but to meet the kids right away.  They're going to come to dad's house and you're going to be living there.  I would not pretend to be just roommates or anything like that.  Children don't like being lied to.

I met my SO's very young kids after we'd been dating just a couple weeks.  He insisted - I didn't think it was a great idea.  We didn't tell them we were dating (nor did we tell them we weren't), and a few of SO's other friends were around.  The first few times I met the kids there were others around and we did fun things.  By the time he told them were were dating they were used to me and they liked me.    I don't think we could have done that had I been living with him.   His kids were also little - the oldest was a couple years younger than your SO's youngest. 

I wish you luck.  You're sort of committed before you know if he's good with his kids or if his kids are well behaved and will accept you.  I hope he is, they are and they will - it'll make things a lot easier.  

Your BF should tell them ASAP and without you there. He should say that you are now living in the household and he expects them to treat you with dignity and respect because you are eager to meet them and get to know them. He should let them ask questions and he should tell them any meaningful information about you -- like that you have a dog or a child, or what you do for a living, or where you met.

Nothing good can come from trying to buy them off with a fun weekend only for them to find out that you are suddenly part of the package. I wouldn't like that dumped on me, so why should they? They will feel betrayed when they realized the weekend was just to cover your bombshell. And you shouldn't be there when they're told -- the kids might be (deservedly) upset and they deserve to have privacy for their meltdowns, questions or otherwise bad reactions to the news. It also will do YOU no good to hear what they might say about this decision -- that's just setting you up to see them at their worst and maybe hate them, and it sets them up to not react honestly if you are sitting right there when you BF tells them.

I think it was a selfish mistake to move in together without considering the kids at all. This is their household too and it was pretty selfish to handle it this way. (I know, I know -- you had to move. Where would you have gone if you didn't have a BF to shack up with? Yea, that's where you should be going now.)

They will react as they react and there's little you can do about it now except to consider them as much as possible at this late date. Don't just spring it on them. Don't try to cover it up with a trip to Disney World. It won't work. They're smarter than you think, and surely more powerful than you realize...but you'll find out soon enough.

Good luck.

Don’t delay, don’t manipulate.  Stick to the facts.  The whole coddling and lying in an attempt to “protect” people is dishonest, manipulative and wrong.  

Be a couple, manage the blended family situation together and don't let the X or the kids interfere in your equity life partnership.

Facts are neither good more bad.  There are merely facts.  Stick to them.