tricentennial (adj.)

word-forming element meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek treis, trias "three" (see three).

"consisting of or lasting 100 years, happening every 100 years," 1789, from Latin centum "one hundred" (see hundred) + ending from biennial. As a noun, "a hundredth anniversary celebration," from 1876; the older noun is centenary.