Triad
mid 16th century: from French triade, or via late Latin from Greek trias, triad-, from treis ‘three’. Sense 2 is a translation of Chinese San Ho Hui, literally ‘three unite society’, i.e. ‘triple union society’, said to mean ‘the union of Heaven, Earth, and Man’.
wiktionary
From Latin triad-, stem of trias(“three, triad”), from Ancient Greek τριάς(triás).
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.
etymonline
triad (n.)
1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados) "a triad, the number three," from treis "three" (see three). Musical sense of "chord of three notes" is from 1801. Related: Triadic.
