trilingual

from WordNet (r) 3.0 (2006)

trilingual
    adj 1: using or knowing three languages; "trilingual speakers"

from The Collaborative International Dictionary of English v.0.48

Trilingual \Tri*lin"gual\, a. [L. trilinguis; tri- (see {Tri-})
   + lingua tongue, language. See {Lingual}.]
   Containing, or consisting of, three languages; expressed in
   three languages.
   [1913 Webster]

         The much-noted Rosetta stone . . . bears upon its
         surface a trilingual inscription.        --I. Taylor.
   [1913 Webster]