The Collaborative International Dictionary
Triennially
Triennially \Tri*en"ni*al*ly\, adv. Once in three years.
Wiktionary
triennially
adv. Occurring at intervals three years apart.
Usage examples of "triennially".
Parliaments elected annually according to the Charter, so many points in which you have carried out, or triennially like the Australians, or septennially as in England of old?