Triplets are groups of three lines of poetry that make stanzas that may have more specific designations, such as tercets or terza rima. My own preference (agreeing with Paul Fussell) would be to use "triplet" as we do "couplet," to designate three lines with identical rhyme, and use "tercet" for the generic three-line stanza with any--or no--rhyming pattern. But the present consensus seems to choose otherwise.
                         I

            Among twenty snowy mountains
            The only moving thing
            Was the eye of the blackbird.

                      II

            I was of three minds
            Like a tree
            In which there are three blackbirds.
                                        --Wallace Stevens

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