Triennial
Triennial , adjective
[Latin triennium the space of three years; tri- (see Tri-) + annus year. See Annual.]
1.
Continuing three years; as, triennial parliaments; a triennial reign. — Howell
2.
Happening, coming about, or appearing once in every three years; as, triennial elections; a triennial catalogue; a triennial visitation. — T. Warton
Triennial , noun
Something which takes place or appears once in three years.