The Mists of Avalon, by Marion Zimmer BradleyRead about the other entries on the list.
Possibly the ultimate fantasy love triangle in the ultimate fantasy novel: the story of Arthur’s rise to the throne of Camelot, told by the women surrounding him from his priestess half-sister Morgaine to his Queen Gwenhwyfar, who had a forbidden love for Lancelet, one of his knights. This novel is controversial and deals with issues such as religious extremism and incest, but at its heart are complex, dramatic relationships as the women navigate love and happiness, which often do not go hand in hand.
Also see Ewan Morrison's top ten literary ménages à trois.
--Marshal Zeringue