Meaning

noun the cardinal number that is the sum of one and one and one

noun a set of three similar things considered as a unit

noun three people considered as a unit

originPIE word *tréyes From Latin triad-, stem of trias (“three, triad”), from Ancient Greek τριάς (triás). By surface analysis, tri- + -ad. Sense 3 (“branch of a Chinese underground criminal society”) is due to the word being applied by the British authorities to underground society in Hong Kong based on the geometry of the Chinese character, derived from a name used by some of those societies, 三合會…

Etymology from Wiktionary, CC BY-SA 3.0