The Collaborative International Dictionary of English v.0.48:
Three-sided \Three"-sid`ed\, a. Having three sides, especially three plane sides; as, a three-sided stem, leaf, petiole, peduncle, scape, or pericarp. [1913 Webster]WordNet (r) 3.0 (2006):
three-sided adj 1: having three sides; "a trilateral figure" [syn: trilateral, triangular, three-sided]