A unique modification of a love triangle involving three partners that all love the other two equally.
Due to the fact that there are only two genders (usually), this must involve at least two bi/gay people to be truly equilateral. However, it's possible for three of the same gender to work, or for all three to be bisexual.
Dude, did you hear? Bill, Jenn, and Michelle are forming an equilateral love triangle!
Bill, that lucky bastard...