Three consecutive rhyming verses are called triplets, as two consecutive rhyming lines are called couplets. In the Restoration and eighteenth century, pentameter couplet verse was sometimes varied by the introduction of the occasional triplet; the third line was often (but not always) an Alexandrine.

A triplet can also be a term used in rhetoric — ???

From the Guide to Literary Terms by Jack Lynch.
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Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.