triennial

English Thesaurus

1. the 300th anniversary (or the celebration of it) (noun.time)

hypernym	:	anniversary, day of remembrance,
definition	:	the date on which an event occurred in some previous year (or the celebration of it) (noun.time)

2. occurring every third year or lasting 3 years (adj.all)

similar	:	periodic, periodical,
definition	:	happening or recurring at regular intervals (adj.all)