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1650s, "three successive lines of poetry," from triple; perhaps patterned on doublet. Extended to a set of three of anything in 1733, and to three children at the same birth in 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 trip·let (trĭp'lĭt)
Any of three children delivered at the same birth.
A set of three similar objects, such as a compound lens in a microscope formed of three planoconvex lenses.
A unit of three successive nucleotides in a DNA or RNA molecule that codes for a specific amino acid; a codon or anticodon.