A tristich is any strophe, stanza, or poem that consists of exactly three lines.
Forms of Tristich
Tristich Parallelism in Hebrew poetry is an early example of this form. The first verse of the book Lamentations being a prime example, with an elegiac sentiment in a progressive parallelism that carries the thought of the first line and adding something thereto.
Shakespeare was another poet who used the tristich form within his Threnody poem The Phoenix and the Turtle.
References
Tristich Wikipedia(Text) CC BY-SA