Here's why:
* Tri- is a prefix meaning "three".
* Tripod is the most common term for a three-legged object, often used for cameras or lighting.
* Trestle is another term for a three-legged support structure, but it's usually used in a more architectural context.
Therefore, a three-legged stool, table, or altar could be referred to as a tripod stool, tripod table, or tripod altar, respectively.