Hi! Is you haven't checked it out already, allow me to recommend this article by the always-smart Franklin Veaux. It's designed for someone coming in as the third in an existing relationship, but I think it's immensely valuable for couples in that configuration as well:
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.