Noun
OT3 (plural OT3s)
(fandom slang) One true threesome; a polyamorous ship involving three characters that is a personal favorite of a fan.
Atack, p. 32 The discovery of OT3 was said to have taken a major physical toll on Hubbard, who announced that he had broken a knee, an arm, and his back during the course of his research. Source: Internet