1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados), from treis "three" (see three). Musical sense of "chord of three notes" is from 1801.
triad tri·ad (trī'ād', -əd)n.
A collection of three things or symptoms having something in common.
The transverse tubule, and the terminal cisternae on each side of it, in a skeletal muscle fiber.