Most often found between 3 gay men, but could also be (in order of likelihood) 2 men and 1 woman, 2 women and 1 man or 3 women.
A Triad is a partnership of equals and does not have the same stigmas associated with it as does polygamy which is sometimes (not always) more about a man married to and/or controlling multiple women who are not involved with each other sexually.
David and Cynthia have been married for a while, but Mike just moved in and they are now a triad.
Person 2: Yeah dude, I'm part of the echelon!
Person 2: That's Thirty Seconds to Mars, that's the triad, and, yes, this is a cult.