I'm also new to this forum, semi-new to poly. I'm in a triad with a married couple (long distance for now) but it didn't begin that way. First, my husband found a girlfriend. She flew out to meet us and she and I fell hard for each other. My marriage ended a few months later--which sounds terrible but in my case it was a really good thing. Having a gf who could provide an outside perspective helped me realize my marriage was emotionally abusive.
My gf has been married 15 years and I slowly got to know her husband. He is very introverted so it took some time. there was never any expectation of me having a relationship with him, and yet he was ready to accept me into his house because his wife's feelings for me were so strong. He was so happy for her to have found me (she has wanted a female partner for a long time). Very recently he and I started to be romantically involved.
For me it was so important that there were no expectations, no pressure from this couple. If it happened, it happened. And if not that was fine. Even now, there isn't an expectation that I will have equal, identical relationships with them both. Although it's important for me to see through my NRE and give time to my gf too.
Anyway, just thought I'd share my perspective. Nice to meet you!