Something that happens every three years is a triennialevent.
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A triennial event happens every three years.
Sextennial means "Recurring every six years."
Biennial means "occurring every two years," OR "lasting for two years." The word biannual can also mean "occurring every two years," although its most common meaning is "occurring twice a year."
Biannual~ this means twice in one yearbiennial~ this means once every two years
Biennial or biannual can refer to something which happens every two years.