The Three Sisters is (um, are?) the name of an ancient farming technique, in which beans, maize and squash were planted in the same place.
The sisters work collaboratively: maize provides a stalk for beans to grow on; beans provide mineral nitrogen for nitrogen-greedy maize; beans and maize together provide shade and humidity for squash; and squash provides weed and erosion control for the other two. And that's only a taste of the benefits of the technique.
Planting beans, squash and maize together was truly a stroke of genius--not to mention a recipe for succotash--and the combination probably dates between 5,000 and 3,500 years ago.