The Collaborative International Dictionary of English v.0.48:

Trio \Tri"o\, n. [It., fr. L. tres, tria, three: cf. F. trio,
   from the Italian. See Three.]
   1. Three, considered collectively; three in company or acting
      together; a set of three; three united.
      [1913 Webster]

            The trio were well accustomed to act together, and
            were linked to each other by ties of mutual
            interest.                             --Dickens.
      [1913 Webster]

   2. (Mus.)
      (a) A composition for three parts or three instruments.
      (b) The secondary, or episodical, movement of a minuet or
          scherzo, as in a sonata or symphony, or of a march, or
          of various dance forms; -- not limited to three parts
          or instruments.
          [1913 Webster] Triobolar

WordNet (r) 3.0 (2006):

trio
    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: a musical composition for three performers
    3: three performers or singers who perform together
    4: a set of three similar things considered as a unit [syn:
       trio, triad, triplet, triple]
    5: three people considered as a unit [syn: trio, threesome,
       triad, trinity]

Moby Thesaurus II by Grady Ward, 1.0:

186 Moby Thesaurus words for "trio":
   German band, Nachtmusik, Philharmonic, absolute music, adaptation,
   air varie, aleatory, aleatory music, arrangement, band, big band,
   bipartisanship, brass, brass band, brass choir, brass quintet,
   brass section, brasses, callithumpian band, chamber music,
   chamber orchestra, clover, coaction, coadjuvancy, coadministration,
   coagency, cochairmanship, codirectorship, collaboration,
   collaborativeness, collectivism, collusion, combo, commensalism,
   common effort, common enterprise, communalism, communism,
   communitarianism, community, complicity, composition, concert,
   concert band, concord, concordance, concurrence, cooperation,
   cooperativeness, descant, desks, deuce-ace, dixieland band, duet,
   duettino, duo, duumvirate, ecumenicalism, ecumenicism, ecumenism,
   electronic music, ensemble, ensemble music, ensemble singing,
   esprit, esprit de corps, etude, exercise, fellow feeling,
   fellowship, gamelan orchestra, group, harmonization, harmony,
   incidental music, instrumental music, invention, jazz band,
   joining of forces, joint effort, joint operation, jug band, leash,
   mass action, military band, morale, mutual assistance, mutualism,
   mutuality, nocturne, octet, opus, orchestra, orchestration,
   part music, part singing, part song, piece, polyphonic music,
   pooling, pooling of resources, production, program music,
   pulling together, quartet, quintet, ragtime band, reciprocity,
   ricercar, rock-and-roll group, score, septet, septuor, sestet,
   set of three, sextet, shamrock, skiffle band, solidarity, sonata,
   sonatina, steel band, street band, string band, string choir,
   string orchestra, string quartet, strings, study, swing band,
   symbiosis, symphony, symphony orchestra, synergism, synergy,
   team spirit, teamwork, tercet, ternary, ternion, terzet, terzetto,
   theme and variations, three, threesome, tierce, trefoil, trey,
   triad, trialogue, triangle, tricorn, trident, triennium, trihedron,
   trilogy, trimester, trine, trinity, trinomial, triphthong, triple,
   triple crown, triple threat, triplet, triplex, triplopy, tripod,
   triptych, trireme, triseme, triskelion, trisul, triumvirate,
   triune, triunity, trivet, troika, united action, variation, waits,
   woodwind, woodwind choir, woodwind quartet, woodwinds, work