triannually

Tributes were required annually, biannually, or triannually, as determined by the distance to Beijing.

Between Winds and Clouds: The Making of Yunnan (Second Century BCE to Twentieth Century CE) 2008

It meets triannually, bringing together the Sunday-school specialists from various denominations.

An Outline of Baptist History: A Splendid Reference Work for Busy Workers. A Record of the Struggles and Triumphs of Baptist Pioneers and Builders N. H. Pius 1911

Well, it's a well-known fact, Sonny Jim, that there's a secret society of the five wealthiest people in the world, known as The Pentavirate, who run everything in the world, including the newspapers, and meet triannually at a secret country mansion in Colorado, known as The Meadows.

Pharyngula 2009