Source : Webster's Revised Unabridged Dictionary (1913)

Three-sided \Three"-sid`ed\, a.
   Having three sides, especially three plane sides; as, a
   three-sided stem, leaf, petiole, peduncle, scape, or
   pericarp.

Source : WordNet®

three-sided
     adj : having three sides; "a trilateral figure" [syn: {trilateral},
            {triangular}]