Term: trilateral

trilateral (adjective):

• General: involving three groups, parties, or nations.
• Geometry: having three sides (though “triangular” is more common in everyday English).

synonyms: three-way, tripartite, triple. 

pronunciation: /traɪˈlæt.ər.əl/ 

translation: trīspusējs 

example: The leaders of the three neighboring countries met to sign a trilateral agreement regarding water rights for the shared river. 

other words from this root: lateral (adjective), bilateral (two-sided), quadrilateral (four-sided).