triennially Synonyms

biennially every three-year event every three-year eventuality every three-year happening every three-year instance every three-year interval every three-year occasion every three-year occurrence every three-year occurrence thereafter every three-year period every three-year phenomenon every three-year cycle every three years every third year thereafter every other year every second or third year every fifth year every fourth year every three years thereafter every second year every sixth year every third year every third year following every third year subsequently every three-year sequence