Knowing ones metamours makes a huge difference to the harmony of a tribe in my opinion. I think I would go out of my way to meet her as soon as possible and by any means possible. During the meeting I think I would express how I feel about it all and ask for in put around what boundaries they and you want to set. You don't have to agree right away, but agree to think about what they each suggest.
Likely the other two are just as anxious and concerned about the result as you. That means you can go into this with empathy and with the approach that they all have needs just as you do. Your gf has been waiting around for some time to be mono with you by the sounds of it. I think I would let her play this out and be their at the end of it or at least until its evident that this woman is a keeper to her. By that I mean about a year and long after the NRE after all.
It sounds like she might be mono too, or at least would prefer that, so I think I would be reassuring her all along that things have changed now and when she is ready, you will be waiting to be exclusively with her if that is what she wants. Thern wait.
As to sex? Its up to you. You don't have to do anything you don't want to do and there is no protocol.
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