He is setting up a bad situtation for everyone. His actions with his daughter now are already determining how well the rest of the relationships in the family with one another will go. I'd have a sit down meeting with him and tell him how it's hurting everyone including her. If he can't see the light then get your kids and get out of there. Even if he agrees with you and says he will really work on it I would still be ready to run with my kids when he doesn't live up to his word. This is what I would have done if I'd known then what I know now. I really hope whatever decisions you make that it all works out for the best.

The child is only an issue because her father allows her to be.  Unless he steps up and properly parents this child, she is always going to behave the way she does and will likely become even worse as she gets older.  If you don't like the way this 3 year old behaves, then you don't need to marry her father becuase her behavior is a direct result of his parenting (or lack of).

He is setting up the household for failure. SD should be expected to respect you and what you ask in the household. You are being pinned as the "bad guy" every time she is allowed to run to him instead of listening to what you say. Now think of her at the age of 17 being able to do this. 

A few questions, if I may. How old are your three kids? Do they do parenting time with their father or solely with you?

I got the idea that one of yous is age three , same as the 2bSD. How often does 2bSD see her father (meaning does she live in your home fulltime, 50/50 , every other weekend and Wednesday evenings, et? 

When your DF allows his daughter to have different meals than the rest, who fixes her 'special' meal? Him? 

While I'm coming across quite the nosy person, some of the advice/suggestions sometimes depends on the additional information not really included in your initial post.

How old have you and DF been attempting to blend your family?  Is this the first experience for 2bSD in dealing with other children in the home? And if so, has she attended pre-school/daycare where she has had to 'get-along'? I highly doubt any day/pre-school fees the kiddos bowls of peanut butter with slices of cheese for lunch though. You did mention eating without the rest. Does that mean she will not sit at the dining table with you and your kids? 

Without additional details (no, not your life story, just some basics on your current household make-up and routine), I will say that your DF is going to have to learn to be the parent an actually parent his daughter. He isn't doing her any favors from babying her or allowing her to be the hard to get along with brat. This little girl is just right around the corner from attending school all day. Dad can bet his buns no one is going to kiss this kid's butt when that starts.

Meals are a major issue for many members with their step kids around here. Answers vary and different solutions for different cases. I will also ask what the child is fed at BM's house? I'm guessing bowls of peanut butter and frozen pizza? 

Welcome to StepTalk. Lots of good idea an advice and always many members who either are going through what you are or have done so and know exactly the frustrations you're feeling. 

Tough situation, but I'm just going to throw out there that it can't just be your way or the highway if you want it to work. I agree to a great extent that the kuds should mostly have the same rules and standards of behavior, but your way isn't necessarily the only way. Like the sharing thing. You don't let anything belong to any specific kid, but because he does, doesn't make him wrong. For example, maybe each kid can have a special toy or 2 just for them, but its put away when eveyone is playing together? (You know, so nothing happens to it, wink, wink, or whatever works). Thats just an example. Same behavior standards (age appropriate, of course), but maybe compromise. Some of his rules, some of yours. 

But it does sound like she's a bit of a bratty 3-year old, which isn't uncommon either lol

Others are correct - he needs to address this now as it will get worse.  I have a son who is now 11 and a daughter who is 8.  My husband has a daughter who is 8.  We moved in together about 3 years ago......  My two had been in daycare settings and had a sibling to learn from.  My now SD always had an in home sitter where she was the center of attention and while she has siblings there are 15+ years older so more like caregivers.  I remember the first few months of meals.......I'd make something and she'd cry b/c she didn't want to eat it and her dad would make her chicken nuggets.  My kids learned from very young - you have to try everything; you don't have to like it but try it (if they didn't then they knew how to make themselves a PB&J).  As this was happening my kids would say "why does she get something different?"  My former fantastic eaters became super picky very soon.  I regret not stepping up and saying something earlier.  This is just one example.

Another - toys.  She has a mentality of "What's mine is mine and what's yours is mine" - she'll ride my kids scooters, play their video games, etc. (honestly this wouldn't bother me in the least if she wasn't so territorial of 'her' stuff) yet I heard her say to my BD a few times "you can't play with that because I don't feel like playing with it now and I don't want anyone playing with my stuff without me."  

Baby talk - she's going to be 9 soon and she'll speak in a baby voice when her dad is around.... I pick her up on Tuesdays and it's just she and I in the car for 20 minutes - she speaks to me in a perfectly appropriate voice..  Once her dad gets home, baby voice.  

It's beyond frustrating; have the talk as soon as you can - otherwise not only you, but your kids will grow resentful.

DOES spoil the whole bunch.  Approach biodad and if he gets defensive, RUN LIKE THE WIND!!!

Looking back at all the stepparent skillz I’ve gained, I’d  address all disrespectful behavior towards you, and your kids, and let her dad make rules for everything else (like what she eats) for her.  If she’s bad at home with you send her to daycare. I spent so much time worrying about my SD and rules it ended up moot point, since as a preteen/teen she ended up doing what she wants (and dad still doesn’t give a crap).  Arguing with them and trying to get her in line just took me away from my own kids.  When DH gets home let him baby her.  I get that she’s annoying, I really, really do, but shes asking for attention, and it’s not at an inappropriate time (like after bedtime), just let her have it.

Nothing good comes of marrying a crappy parent.

I didn't get the signficance of this when my bf first laid down the law, but he said all kids had to be treated equally and the same rules apply to everyone. 

I agreed, but didn't totally get it until it was in action. My son was the one who was an only child and who had pretty lax rules. His two kids have a very strict BM and an unreasonably strict previous SM. Now my son has no issue in sharing, he's always been a good sharer, so that wasn't a problem. But he was used to eating what he wanted pretty much and lax table rules. All that changed. 

What SO and I did is we talked  -  A LOT - about rules and approaches. We didn't have these talks in front of kids. Things tightened up on my son. He adjusted. Some things loosened up on his kids. 

Our blended family of 5 is full of awful picky eaters so I do try to make sure that there's something for everyone to eat, but even my veg refusing son has to eat some vegetables. 

You have to establish NEW household rules that are the same for all kids (within reason - they're all individuals) and that are enforced no matter what mix of kids are present in the home. 

He is choosing her over you. She is his “mini-wife” and I strongly advise you to look it up here in the search bar for those posts. He has double standards-he’s treating YOUR kids like kids, but HIS kid as his equal. He expects YOU to parent your kids but also expects YOU to also cater to his kid when it comes to meals and letting the rules slide. Do double standards eat at you? Do you cope well with favoritism in families? Because this is what’s about to happen. Your kids will eventually get fed up with the kid’s special treatment, your boyfriend’s obvious favoritism and you for not defending them. And they will leave you.