(Sorry for the long answer please ignore it if you think it's over the top)

First: It's great that she knows what is good for her for now and she is taking actions to take care of herself. I don't see that as an ultimatum but rather as a way for her to deal with the situation.

Like Ocean Swimmer said: maybe an activity with all 3 kids wil help her to connect (and 2 and 3 years old aren't that hard to get together),
If she refuses to do this or if she cannot connect to them there are a few questions that you have to ask yourself

1) Are you willing to wait if that is going to change in the future and make an agreement that works for you both untill that day
2) Are you aware of the possibility that this may not change?
3) Can you be together with someone who doesn't want to connect to your children?
4) If yes how should this work? She seems to already have her plan on this and has given you the information what works for her, but does that work for you too? In other words: Can you accept for yourself that this is your agreement? Or can you compromise on some topics?
5) How will that affect your children?

I agree that you both are young and that there is so much time for you two. Your relationship is young too and there are so many things to happen and explore at each other. You don't have to make big desicions today or tomorrow. You don't have to answer that questions today either. Take your time. Let her take time. Have you ever asked her if she even wants to be a step-mother? at all?

For the long run I would say you have to think about how/if you explaine this to your children. Will she just not be there? Are you going to tell them why? 3 years old ask a lot of questions and it won't stop soon. How are you handling this now?

I think this can work. Living seperate is no drama and if you have your children 50% of the time, there is plenty of left for your love life and for your children. The question is: Can you live with them to be separated? And how long are you willing to wait/try/work if not?

Wish you luck