A three-stanza poem is a poem divided into three sections, or stanzas. Many famous poems, including A.E. Housman's "Loveliest of Trees," William Carlos Williams' "This Is Just To Say" and Richard Lovelace's "To Lucasta, Going to the Wars" conform to this structure.
Often, the stanza breaks in a three-stanza poem serve to underscore a logical shift. For example, here is Lovelace's "To Lucasta, Going to the Wars":
"Tell me not, sweet, I am unkind,
That from the nunnery
Of thy chaste breast, and quiet mind,
To war and arms I fly.
True; a new mistress now I serve,
The first foe in the field;
And with a sterner faith embrace,
A sword, a horse, a shield.
Yet this inconstancy is such,
As thou too shalt adore;
I could not love thee (Dear) so much,
Love'd I not honor more."
In the first stanza, the speaker makes a request of his lover, asking that she not reprove him for leaving her quiet, domestic side to go to the excitement of war. The second stanza is a concession to a potential counterargument: he acknowledges that he is now serving a "new mistress." This mistress, however, is not a woman, but the necessities of war. The final stanza rebuts that counterargument, however, by reminding Lucasta that she would not love him if he were a dishonorable man who shirked his military duties. The three stanzas in the poem reflect the logical progression of its argument.