BL, I just want to pop in with something on the little one not being a fan of OM. I liked a total of 2 of my mom's bfs. I'm actually still social media friends with one of her exes. But he was probably the best of them all. He still came to see me after they broke up. He wanted to marry my mom but she said she wasn't ready and he didn't want to back track in the relationship. So it ended where it ended, but he never stopped being kind to either of us. And he didn't just leave me behind when they split.
The others, however, some were openly resentful and cold to me. Some just gave me a really bad feeling. Honestly I think kids are a lot more intuitive than adults or they're just better at listening to their gut. My feelings on those guys were never wrong.
That being said my daughter haaatttteed my now husband and my exH's 1st gf after the split. She genuinely did not like me splitting my attention. She had grown accustom to being my whole world. And even though she grew to love my exH's now ex-gf (she was the best of all of them) she didn't appreciate her dad's behavior. He is the kind of guy who gets swept up in a relationship. Not that he was ever particularly active in our daughter's life but he completely dropped off the face of the Earth for the first few months they were seeing each other. She felt like he was always choosing someone or something over her and she (the gf at the time) was no exception.
Another example my high school bf was also part of a blended family, like I was, but in their house they were forced to call the step parent by either mom or dad. That in and of itself bred a ton of resentment for all the kids. The grossest thing about that is that their non-custodial parents (who admittedly were shi!tshows) were still in their lives to the best of their abilities. On top of that they were married when the kids were like 8, 10, 12, and 14. IMO a little too old to play Brady bunch. Also a little to far this side of the 1970s. It literally became a household of kids vs the parents because of the dynamic they created. My house was a nightmare, but that was one horror I was never forced into.
The point here is kids can dislike the dynamic for a plethora of reasons. Maybe your ex behaves differently when OM is around. Maybe D3 just isn't a fan. Maybe D3 isn't getting the attention she wants. Maybe when OM is around they do what OM wants to do and when he isn't your ex let's the kids do what they want to do. I'd keep and eye out for obvious signs, but make sure you keep in mind 3 year olds don't prescribe to logic yet. They still live in the realm of magical thought. And at that age developmentally speaking they are all essentially tiny sociopaths that lack empathy. Not to defend OM but he could be a perfectly acceptable bonus parent and D3 could just like things they way she likes things, in the same manner she probably has a favorite cup.
Lastly, remember that digging into what's going on in the other household opens your household up for the same kind of scrutiny when the time comes. Which I'm sure right now that doesn't mean much. Probably you're thinking, fine, I got nothing to hide. But it gets complicated as they age and if you meet someone you want to stick around. If they don't learn the boundaries of what happens at mom's stays at moms unless you were scared or hurt, and what happens at dad's stays at dad's unless you were scared or hurt early it becomes a whole thing. Trust me I lived/live it with H's ex. D17 still doesn't get the boundaries sometimes. It's exhausting and frustrating when dealing with an ex who is always looking for way to hurt, manipulate or get more money.