Synonyms for Trilateral (same or very similar meaning)
WordNet sense 1 (a three-sided polygon):
triangle, trigonWordNet sense 2 (involving three parties or elements):
triangular, tripartite, three-party, three-wayWordNet sense 3 (having three sides):
three-sidedWordNet sense 4 (having many parts or sides):
many-sided, multilateralFrom the ODE community, based on WordNetadd/edit