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