Needing Help With Plural Marriage

Hi, I am new here and am in need of advice. I apologize because this is a very long and drawn out story but I do not have anyone I can talk with. My best friend and I have been interested in each other for a long time. Five years ago she met a man and started a relationship with him. They have a child together and her and I are still interested in each other. I have recently learned that her husband has always had an intrest in me. I have developed feelings for him as I have for her. I know that he feels the same way about me and neither one of us want to see anyone get hurt. Both he and I want all 3 of us to be a family. My best friend is somewhat open to this idea and wants the 3 of us to sit down and discuss how it would work. She is scared that she will be pushed out of the picture because he and I are so much a like. I really would like advice or tips on how to make this work. I know this is different from the usual plural marriage in the fact that she and I want to be together sexually but that is not what it is all about. We are best friends and I want to spend the rest of my life with her and him. Thanks!