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