triennial

Occurring every third year.

Lasting three years.

A third anniversary.

A ceremony or celebration occurring every three years.

Continuing three years: as, triennial parliaments; specifically, of plants, lasting or enduring for three years.

Happening every three years.

A mass performed daily for three years for the soul of a dead person.

A plant which continues to live for three years.

Any event, service, ceremony, etc., occurring once in three years; specifically, the third anniversary of an event.

Continuing three years.