Yeah don't tell her not to talk about her mom, that is PAS and at this age, she'll probably resent you for it. She probably talks about you guys a lot at her mom's house, it is pretty common at this age. I also think you should change your approach in this conversation.
SD: FTMandSM and mommy don't get along.
Me: That's not something you need to worry about. We both love you and that's what matters most.
SD: Yeah. You are mean to mommy and you don't like her.
Me: I barely know your mommy. I bet there are a lot of things you love about her!

She's 3, and while BM is prob saying things about you out of her own insecurities, there are a lot of things she is probably trying to figure out about her situation and where everyone fits in her life. It is important to make sure she knows the grown-up issues aren't things she needs to worry about and that she is loved in both her homes.

After everything that I have read about PAS, and my own experiences with it, I have come to the conclusion that one of the trickiest things about battling PAS is the line between fighting against it and falling into being guilty of it as well.

I would say to keep researching and know your own personal boundaries. Asking a three year old not to talk about her mom while in her dads care would probably be considered PAS. All that the three year old needs is to know that she is loved at your home and cared about. I like the fakemommy's comment about rewording the conversation a little bit.

There is a talk that I have had with both skids and my bios from young ages up. I would tell them this: There are a lot of different kinds of families. Some families have a mommy and daddy who are married, some have just a mommy or just a daddy, some kids have a lot of siblings and some have none. Some kids have a step mommy/daddy and step siblings or even half siblings. There are some families where kids live with their grandma or aunt. (If you really want to get into it some families have two mommies or two daddies, which is actually what sparked this conversation with me and my skids) The thing is, families can be made up of all different kinds of combinations, but what is important is that the people love each other, look out for each other, teach each other etc. etc. etc. So it might be a nice route to just have a conversation like that out of the blue and not in response to something your SD brings up.

I wouldn't get into it with her. If she says you and bm dont get along, tell her you do. At her age getting alon is equivalent to liking each other is equivalent to bein friends is equivalent to loving Each other. At three she wants everyone to love each other and be friends. Keep it light, positive, happy and move on to something fun.

It could be that BM is talking about you to her friends or family IN FRONT of SD, without thinking that SD is absorbing as much as she is. Many parents of toddlers don't stop and think about what little sponges they are...I've seen this many times first hand with my nieces and their daughters. Hopefully BM will start to realize she's catching on to these things sooner rather than later and just stop. I don't know if SD's dad is brave enough to approach BM about the topic, but maybe he should let BM know what his daughter's been saying and for the sake of SD's healthy emotional development he would like for all 3 of you to be able to get along. And the best way for that to happen is that you all remain civil to, and about, each other in front of the daughter?

I think I would tell her to not talk about mommy unless it is good things.

When she says..."Mommy wants to hit you." Gently remind her that she is not to talk about BM unless it is a nice thing and hitting people is not nice.

"Responsibility" that's an interesting word for a 3 y.o. Maybe you could have asked her to define it to see where she was going with it. I have no doubt this is PAS. Remember, this is the syndrome of the child we're talking about, not the behavior of the parent. While it could have been unintentional on BM's part, it still is creating unnecessary loyalty conflicts which need to be corrected.
My SD17 started showing signs of PAS at 10. At 10.5 she stated "It's not my responsibility to visit you, (DH), you need to come visit me if you want to see me" , which was really out of context for her at the time. Fast forward to now, we are disengaged from her and She's a train wreck. PAS took it's toll on all of us. If we had known how to combat it then, things may have been different. I fell into the same trap as
mentioned above, I pointed out the alienating parents behavior to SD, which backfired. Yes, have DH talk to BM about it, see where it goes, if that doesn't work go straight into counseling. Good luck!

Ya, I read Divorce Casualties, but it was too late to undo the brainwashing. Our attempts only made it worse. Her middle school nurse caught her cutting and ordered therapy, which helped undo the self harming behavior. But she's still so emotionally screwed up it's just plain sad. We just need to wait it out to see if the girl is smart enough to figure it all out. There have been moments of clarity for her, but nothing lasting. You're lucky SD is only three and you have this site and good books to read. You can actually prevent real damage so that your SD can function in the world. Please don't ignore the obvious! There is so much you can do at this point. Nip it!