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Triannual (adjective)
Happening three times each year.
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Triennial (adjective)
Happening every three years.
“triennial elections”
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Triennial (adjective)
Lasting for three years.
“triennial parliaments; a triennial reign”
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Triennial (noun)
A third anniversary.
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Triennial (noun)
A plant that requires three years to complete its life-cycle.
Wiktionary