Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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