The Three Sisters were the three foundational crops of Native Americans, prior to European colonization of the Americas.
In tradition agriculture, maize (corn) was planted (often with some high-nitrogen fertilizer) such as a small fish under the seeds. Besides its own production, it served as a support for the second sister - climbing beans. The third was a member of the squash/pumpkin family, which provided shade for the soil and reduced weed growth.
While the three crop association may seem accidental; the crops, supplemented with a small amount of protein from animal sources provided high quality nutrition of the native population.
See also: Milpa agriculture