triplet (n.)youdaoicibaDictYouDict[triplet 词源字典]
1650s, "three successive lines of poetry," from triple; perhaps patterned on doublet. 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.[triplet etymology, triplet origin, 英语词源]