Learn how to say Triennially correctly in English with this tutorial pronunciation video.
Oxford dictionary definition of the word triennial:
recurring every three years:
the triennial meeting of the Association
lasting for or relating to a period of three years.
a visitation of an Anglican diocese by its bishop every three years.
mid 16th century: from late Latin triennis (from Latin tri- ‘three’ + annus ‘year’) + -al