The Three Sisters are the three main agricultural crops of various [Indigenous] American groups in North America: winter squash, maize (corn), and climbing beans (typically tepary beans or common beans). The Iroquois, among others, used these « Three Sisters » as trade goods.  Source: https://en.wikipedia.org/wiki/Three_Sisters_(agriculture) 

There are several legends surrounding the Three Sisters; indeed, almost every American [Indigenous]  nation seems to have its own. The Cherokee legend involves three women who helped each other stay fed, hydrated and strong on the Trail of Tears, a lesson that the Cherokee used in planting their crops when they arrived in the Oklahoma Territory. Another legend describes three sisters who bickered constantly until their mother gave each of them an egg cooked in a different way and showed the sisters that, although the textures of the eggs were different, they were still eggs.

This is one version of the Haudenosaunee legend of the Three Sisters:  (Read the more at The Legend of the Three Sisters on The Oneida Indian Nation site at http://www.oneidaindiannation.com/the-legend-of-the-three-sisters/)

 

The three sisters planted together:

The Iroquoians got most of their food from farming corn, squash and beans. By planting them together in the same field, these three plants can help each other grow. For example, the beans use the corn stalks for climbing and the corn leaves protect the squash from the wind and sun. These three products are called the three sisters.

 

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Trois sœurs – Les trois sœurs sont une légende que les Iroquois ont transmise aux Européens. Les trois sœurs sont les haricots, les courges et le maïs. La légende enseigne que ces sœurs se portent mieux lorsqu’elles sont cultivées ensemble.