I don't think either of us have ever doubted that she is the only person we've met who seems like a person who we would be willing to open up for, we mesh really well.

Meshing is one thing, but looking at her dating life and her complaints, I'm not sure about that.

So, while the polyamorous portion is new and scary, she knows us very well and knows what to expect.

How does this make things any easier? She knows you, but neither you, nor your gf, nor her have ever had a polyamorous relationship.

Our intention is to make it as loose of an arrangement as possible to stress that she doesn't have to feel obligated to call us partners, at least until she is ready. If she chooses one of us over the other, I think that's a conversation worth having. If she chooses my GF, I'm willing to sit back and let it play out because that would be a first real female romance for both of them. If she chooses me, that's something that I would be genuinely concerned about.

But what about your gf and you then? Wouldn't this mean that you're no longer primary partners? What if jealousy comes up? You really need to discuss this more.