WordNet (r) 3.0 (2006):

threesome
    n 1: the cardinal number that is the sum of one and one and one
         [syn: three, 3, III, trio, threesome, tierce,
         leash, troika, triad, trine, trinity, ternary,
         ternion, triplet, tercet, terzetto, trey, deuce-
         ace]
    2: three people considered as a unit [syn: trio, threesome,
       triad, trinity]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "threesome":
   bout, clover, contest, decathlon, deuce-ace, double-header,
   doubles, event, foursome, game, games of chance, go, leash, match,
   meet, pentathlon, play, play-off, race, runoff, set of three,
   shamrock, singles, sport, tercet, ternary, ternion, terzetto,
   three, tierce, trefoil, trey, triad, trialogue, triangle, tricorn,
   trident, triennium, trihedron, trilogy, trimester, trine, trinity,
   trinomial, trio, triphthong, triple, triple crown, triple threat,
   triplet, triplopy, tripod, triptych, trireme, triseme, triskelion,
   trisul, triumvirate, triune, triunity, trivet, troika, twosome