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]