The premise of the three sisters is that each element works together for maximum yield with minimum pest and efficient use of space. When interplanted together, corn stalks work as poles for the beans which add nitrogen to the soil feeding the corn and the squash provide ground cover retaining soil moisture. Each confuses ore frustrates the pest generally drawn to the other supporting organic practices.
Corn is planted in rounded mounds a foot high and 24 inches across with flattened tops (each center four feet apart). Five or six kernels are planted in a small circle around the mound. After a few weeks when the corn is about five inches high, beans are planted six inches away from the corn. A week later on the side of the mound seven or eight squash or pumpkin seeds are planted.
Admittedly, I don't follow the layout exactly. My corn is planted in traditional rows with kernels planted 24 inches apart (rows are spaced walking distance apart). Beans are planted between each corn shoot when the corn is five inches high. A week later I plant pumpkins or melons offset from the beans every five feet. This has been my practice for several years now and I really like the way it comes together. Once the harvest is complete, my herd of goats graze the stalks and vines alternately with the chickens cleaning up as well.
If you have children at home, consider adding a three sister's garden as a unit study in garden practices or native American culture. My children loved studying the gardening methods of other cultures..it's amazing how ingenious people can be.