The Jonas Brothers, e.g (4)
(Other definitions for trio that I've seen before include "Piece for three players" , "Three together" , "Three of a kind" , "Band of three" , "wise men?" .)
(Other definitions for threesome that I've seen before include "One more than a couple" , "Horse meet (anag.)" , "Chekhov's sisters" , "triangular event" , "Blind mice, perhaps" .)