Sunday Bloody Sunday (1971)

Recently divorced career woman Alex Greville begins a romantic relationship with glamorous mod artist Bob Elkin, fully aware that he's also intimately involved with middle-aged doctor Daniel Hirsh. For both Alex and Daniel, the younger man represents a break with their repressive pasts, and though both know that Bob is seeing both of them, neither is willing to let go of the youth and vitality he brings to their otherwise stable lives.

Source: TMDb