Definition from Wiktionary, the free dictionary
triennial (not comparable)
- Happening every three years.
- triennial elections
- (Can we find and add a quotation of T. Warton to this entry?)
- Lasting for three years.
- triennial parliaments; a triennial reign
- (Can we find and add a quotation of Howell to this entry?)
happening every three years
lasting for three years
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See also 
triennial (plural triennials)
- A third anniversary.