Wiktionary
triannually
adv. 1 (context uncommon English) Three times per year. 2 (context proscribed colloquial English) Occurring every three years; triennially.
Usage examples of "triannually".
Maynard Allardeck, acting as Steward already at several big meetings, was aiming for the triumvirate, the three Stewards of the Jockey Club, from among whom the Senior Steward was triannually elected.