PLove, I chime in with Mr...Right. Although maybe not a girls' night out, but a girls' weekend out! Do you feel there might be an interest or hobby you share with her that you could pursue together, or would you be willing to develop a one?
I think one of the biggest hurdles in the way of most triads is the idea that WE NEED TO DO EVERYTHING TOGETHER! Or otherwise it's no triad.
Wrong! Although it might feel most comfortable, staying on your comfort zone is basically no longer an option if you are a previously monogamous couple who has decided to explore polyamory. Interacting with two people at the same time, even if it's just talking about inconsequential things, takes up way more energy than one-on-one. Especially if you all the time have to guess at whether somebody is feeling left out, or if the two others are secretly wishing you'd just go away so they can get to it.
Spend relaxed no-pressure time doing something else than processing with each of you partners individually is my advice. Part of the beauty of polyamory for me is that you don't need to like all the things your partner likes and want all the things they want, because both of you can have your needs met elsewhere. This is not an excuse to have a shoddy foundation, but I do find it takes some of the pressure out of dating at least when you don't have to constantly do calculations in your head on whether this is a person you are more compatible with than any other person in the world could ever possibly be.
Me: bi female in my twenties