Eternal Triangle Meaning

(idiomatic) A relationship involving three persons (usually two women and one man or two men and one woman) among whom there are conflicting and competing attachments of a romantic or emotional nature.

Example: 1920, D. H. Lawrence, Women in Love, ch. 31:
  "There weren't even any words," she said. "He knocked Loerke down and stunned him, he half strangled me, then he went away." To herself she was saying: "A pretty little sample of the eternal triangle!"
1963 Sept. 6, "Cinema: After the Money Rolled In," Time:
  Wives and Lovers perkily proves that the eternal triangle is still good for laughs. The triangle consists of Author Van Johnson, Wife Janet Leigh and Literary Agent Martha Hyer, who sells Van's novel to a publisher, a book club, a Broadway producer and a movie company. . . . Not content with her 10%, she tries to collect the author too.
2006 April 6, Margaret Pomeranz, "She's The Man," (Australia) (retrieved 4 June 2013):
  And then there’s the eternal triangle thing with Duke fancying Olivia who fancies Sebastian.