1640s, "relation to or consisting of 100 years," from Latin centenarius "of a hundred, relating to a hundred," from centenai "a hundred each," from centum "hundred" (see hundred).
As a noun, c. 1600 as "period of 100 years;" 1788 as "a hundredth anniversary, commemoration or celebration of a hundredth anniversary." The usual British word in this sense for the American centennial.
<a href="https://www.etymonline.com/word/tercentenary">Etymology of tercentenary by etymonline</a>
Harper, D. (n.d.). Etymology of tercentenary. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/tercentenary