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:
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.