triad

1. a group or set of three related people or things.

   the triad of medication, diet, and exercise are necessary in diabetes care

2. a secret society originating in China, typically involved in organized crime.

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’.