How about "Freida" about the life of Frieda Kahlo and Diego Rivera. They were not polyamorous, but they sure had many loves outside of their marriage. I friggin love Selma Hayek in this role.
Another true story along the same lines I can think of is "Henry and June" based on the diaries of Anias Nin, who struggled with her balancing act between many loves including her husband, cousin, and Henry Miller and his wife June. (The book is waaay better,