Unfortunately the list of movies and books of consensual non-monogamy (CNM) in a contemporary setting is short. In the 1970s, when I was young, that list was dismal. Paint Your Wagon was a standout but another title that deserves honorable mention is Lovin' Molly (1974).
In today's vernacular you'd call the story an MFM triad of two brothers in a polyamorous romantic and sexual relationship with one woman that was (um-mostly) consensual. It is based on Larry McMutry's lesser known 1962 second novel "Leaving Cheyenne" and the movie almost works. It has a star studded cast that includes young Anthony Perkins, Beau Bridges, Blythe Danner and Susan Sarandon. It's worth a watch even though it sadly falls short of its potential. That's kinda understandable given there was no real narrative for functional contemporary CNM at the time.
