1
general::
A relationship involving three lovers, such as two women involved with one man or two men with one woman. For example, The plot of the murder mystery revolved around the eternal triangle of a husband, wife, and another woman. [c. 1900]
American Heritage Idioms
2
general::
Phrase(s): the eternal triangle
a sexual or emotional relationship involving two women and one man or two men and one woman. (Typically, a couple [man and woman] and another man or woman.) • Henry can’t choose between his wife and his mistress. It’s the eternal triangle. • I’m surprised Jane doesn’t get tired of the eternal triangle. She goes out with Peter at the weekend and Jim during the week.
McGrawhill's American Idioms And Phrasal Verbs