"Tri" is a Latin prefix meaning "three". It appears in many English words that refer to groups of three, such as:
* Triangle: a three-sided shape
* Tricycle: a three-wheeled vehicle
* Tripod: a three-legged stand
* Tripartite: consisting of three parts
* Trifecta: winning the first three places in a race
So, "triad" literally means "a group of three".