Love triangles hardly ever turn out well— not in real life, or mythology either. The Glaucus/Scylla/Circe triangle leaves Scylla a horrible monster, Glaucus really sad, and Circe... well, Circe doesn't really care that much. In any case, the tale of "Glaucus and Scylla" is far from the only myth to include highly unfortunate love triangles.
Arthur, Lancelot, and Guinevere
One super famous love triangle comes from the court of King Arthur. You've probably heard of this guy, right? He was an awesome king who invented chivalry and recruited a bunch of noble knights to bring justice to England. Trouble came, though, when his best knight, Lancelot, and his gorgeous wife, Guinevere, fell in love. In many versions of the story, the affair that resulted ended up destroying Arthur, the Knights of the Round Table, and Camelot itself. For more on this tale, check out Le Morte D'Arthur by Sir Thomas Mallory.
King Mark, Tristan, and Iseult
Some think that this popular medieval tale might have actually influenced the Arthur/Lancelot/Guinivere story. In this myth, King Mark's trusted nephew and awesome knight, Tristan, is sent to Ireland to bring Iseult back as a bride for Mark. Unfortunately, Iseult and Tristan accidentally drink a love potion on the way back to England and fall in love. Their secret passion ends up causing all kinds of problems, in many versions leading to all of their deaths.