WordNet sense 1 (the cardinal number that is the sum of one and one and one):

leash, ternary, three, trey, trinity, trio, triplet, troika, deuce-ace, III, tercet, ternion, terzetto, tierce, trine

WordNet sense 2 (a set of three similar things considered as a unit):

triple

WordNet sense 3 (a three-note major or minor chord; a note and its third and fifth tones):

common chord