WordNet sense 1 (a set of three similar things considered as a unit):

triad, trio, triplet

WordNet sense 2 (three times as great or many):

threefold, treble, three-fold

WordNet sense 3 (having three units or components or elements):

ternary, triplex

WordNet sense 4 (having or involving or consisting of more than one part or entity or individual):

multiple

WordNet sense 5 (a base hit at which the batter stops safely at third base):

three-bagger, three-base hit