I first heard about the Three Sisters on a cooking show.  Native American indians in many different regions of North America used to plant corn, pole beans, and squash together.  Although many different Native American people have adopted this traditional gardening technique, it originated with the Haudenosaunee (hah-dee-no-show-nee), or "People of the Long house".  There is an abundance of folklore, stories, and history surrounding Three Sisters gardening.  By using this companion planting, each crop provides benefits for the other two crops and you can grow more in a small space. 

The corn grows and provides a "pole" for the beans to run up and shades the squash.  The squash covers the ground around the corn and beans reducing the need to weed and provides even moisture for the corn and beans to develop better.

The first time I grew the three sisters, I planted all three at the same time.  I placed 2 corn kernels, 3 beans, and 2 squash in each mound.  The corn only grew half height and the ears did not mature.  The beans didn't grow as much as normal.  And the squash only produced one fruit.