Tres Horny Boys refers to the three main characters of the Balance Arc Campaign:
History[]
This nickname comes from the 3rd Eleventh Hour episode. When deliberating on a name for the three of them, Justin goofily said "Tres Horny Boys" but immediately responded to the name with "nobody ever say that out loud ever again please." Sadly (for him at least), as Griffin explains, "everything you say on this show becomes canon."