Tri- means three
triangle (n) - a three-sided figure
triathlon (n) - a race that combines three activities - swimming, bicycling, and running
triceratops (n) - a dinosaur with three horns - two long horns above the eyes and one short horn on the nose
tricycle (n) - a vehicle with three wheels
trilateral (adj) - having or involving three sides, countries, or parties
trilingual (adj) - able to speak three different languages
trilogy (n) - three related plays or novels (or movies)
trio (n) - a group of three people or things
tripod (n) - a three-legged stool, table, or stand used to hold things, such as a camera
trisect (v) - to cut into three parts