From Latin triennisfromtrienniumtrienniumtriennium
From
American Heritage Dictionary of the English Language, 5th Edition
From Latintriennium (“three-year period"), from tres (“three") + annus (“year").
From
Wiktionary
Triennial Sentence Examples
Finally a great triennial competition decides the elections.
His connexion with this work so enhanced his popularity that he was chosen governor by an overwhelming majority and served for two triennial terms (1817-1823).
Most of the assemblies were annual, some triennial, some lasted only a day or two, others a week and occasionally longer.
He maintained the connexion of church and state, and opposed triennial parliaments and the ballot.
In 1693 Temple sent him to try and convince the king of the inevitable necessity of triennial parliaments.