tercet
noun
ter·cet ˈtər-sət 
: a unit or group of three lines of verse:
a
: one of the 3-line stanzas in terza rima
b
: one of the two groups of three lines forming the sestet in an Italian sonnet
Word History
Etymology
Italian terzetto, from diminutive of terzo third, from Latin tertius — more at third entry 1
First Known Use
circa 1598, in the meaning defined above