1650s, "three successive lines of poetry," from triple; perhaps patterned on couplet. Extended to a set of three of anything by 1733, and to three children at the same birth by 1787 (another word for this was trin, 1831, on the model of twin). Musical meaning "three notes played in the time of two" is from 1801.
Proto-Indo-European root meaning "to fold."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit putah "fold, pocket;" Albanian pale "fold;" Middle Irish alt "a joint;" Lithuanian pelti "to plait;" Old English faldan "to fold, wrap up, furl."
popular name of a German-American Anabaptist sect, 1756, from (Pennsylvania) German Tunker, from tunken, dunken "to dip, soak" (see dunk (v.)). So called because they practice adult baptism by triple immersion. The proper name is Brethren.