From Longman Dictionary of Contemporary Englishtriennialtri‧en‧ni‧al /traɪˈeniəl/ adjective TMChappening every three years → annual
Examples from the Corpus
triennial• At their triennial congress on Sept. 15-21,1990, delegates adopted a new party programme replacing that of 1975.• June 30 would mark the end of the triennial contract cycle that had punctuated labor-management relations in the copper industry since midcentury.• The triennial election for the Senate Presidency took place on Oct. 2.• The Fourth Lateran Council, held at the end of his reign, imposed a triennial twentieth for the same purpose.Origin triennial (1500-1600) Latin triennis, from triennium “three years”, from tri- + annus “year” 