My husband, his girlfriend and I recently watched the 1962 film Jules and Jim. Here is the Netflix synopsis:
Writers Jules (Oskar Werner) and Jim (Henri Serre) are close friends who fall in love with the same woman, the unpredictable Catherine (Jeanne Moreau), amid the turbulence of World War I Paris in one of director François Truffaut's best-loved films, adapted from the novel by Henri-Pierre Roché. What results is a decades-long love triangle that both tests and strengthens the bond between the two men.
She is married to one of them and openly seeing the other for most of the film. I was really impressed by the woman's husband's ability to accept her need to see the other guy. At one point something was said about "inventing a new form of love" or being "pioneers of love," I wish I could remember the exact quote.