The classic "eternal triangle" is
Sartre's, in
No Exit. Briefly, the play defines
hell as an eternally locked room holding three people, each in love with one who can't stand her/him, and irritated by the one who loves her/him.
Googling "eternal triangle" results in a funny range - from economic to geographic to emotional. I followed one link to find the following definition: "an emotional relationship in which there are conflicts involving a man and two women or a woman and two men". This definition is so distracted by the sexes of the participants (which are irrelevant) that it downplayed the points of the phrase, which are romance and conflict. I had to laugh.
I am currently participating in a triangle but I wouldn't call it an "eternal" triangle, partly because of issues with the concept of "forever," but more because it's somewhere in between a conflict-of-three and polyamory. Calling it an "eternal triangle" would seem to doom one or more of us to ongoing unhappiness. I believe that non-traditional relationships can work.