Synonyms for Triangular (same or very similar meaning)
WordNet sense 1 (having angles or an angular shape):
angular, angulate
WordNet sense 2 (involving three parties or elements):
trilateral, tripartite, three-party, three-way
WordNet sense 3 (having three sides):
three-sided
WordNet sense 4 (having many parts or sides):
many-sided, multilateral
From the ODE community, based on WordNet
add/edit