three
tri-
(n) a long-distance race that has three parts (usually swimming, bicycling, and running)
triathlon
(n) a series of three novels, movies, etc. that are closely related and involve the same characters
trilogy
(n) a shape made up of three lines and three angles
triangle
(n) a three wheeled vehicle that is moved by foot pedals
tricycle
(n) a support or stand for a camera, telescope, etc. that has three legs
tripod