I have to say I am impressed by your interest and your initiative. I have never been a "3rd" but I was dating one for a few months. Once it got to serious she backed away. She doesn't and won't consider a relationship as my wife and I would love to have with her. Obviously thats her choice, and I can't fault her.
Reading this, while it hasn't worked out to this point, I am happy to see people interested in at least understanding. While I am still not convinced I am 100% poly (I am at the stage where I think this wonderful woman was likely a 1 off piece of chance, feels almost like a cosmic tease haha) I enjoy knowledge and perspective. Poly has introduced something very important to any relationship, full disclosure communication. Even examing poly as a potential lifestyle has greatly improved my relationship, hopefully you can walk away with a life lesson you can carry into future relationships