What is a three stanza poem?

a stanza is a unit within a larger poem. A good example of a 3 stanza poem is Keats' "To Autumn"

Check the link for an analysis for why it is a good 3 stanza poem.

Season of mists and mellow fruitfulness

Close bosom-friend of the maturing sun

Conspiring with him how to load and bless

With fruit the vines that round the thatch-eaves run;

To bend with apples the moss'd cottage-trees,

And fill all fruit with ripeness to the core;

To swell the gourd, and plump the hazel shells

With a sweet kernel; to set budding more,

And still more, later flowers for the bees,

Until they think warm days will never cease,

For Summer has o'er-brimm'd their clammy cells. (lines 1-11)

Who hath not seen thee oft amid thy store?

Sometimes whoever seeks abroad may find

Thee sitting careless on a granary floor,

Thy hair soft-lifted by the winnowing wind;

Or on a half-reap'd furrow sound asleep,

Drows'd with the fume of poppies, while thy hook

Spares the next swath and all its twined flowers:

And sometimes like a gleaner thou dost keep

Steady thy laden head across a brook;

Or by a cider-press, with patient look,

Thou watchest the last oozings hours by hours. (lines 12-22)

Where are the songs of Spring? Ay, where are they?

Think not of them, thou hast thy music too,-

While barred clouds bloom the soft-dying day,

And touch the stubble-plains with rosy hue;

Then in a wailful choir the small gnats mourn

Among the river sallows, borne aloft

Or sinking as the light wind lives or dies;

And full-grown lambs loud bleat from hilly bourn;

Hedge-crickets sing; and now with treble soft

The red-breast whistles from a garden-croft;

And gathering swallows twitter in the skies. (lines 23-33)

A three verse poem