Planting the "three sisters" (corn, beans and squash) together promotes good health for each. Beans provide nitrogen for all three, but especially the nitrogen-greedy corn. Corn also needs lots of moisture, which the large squash leaves preserve by keeping the soil shaded. The corn, in turn, provides a natural support for the beans and additional cover for the squash in very hot climates. The beans also keep the three "bundled" together, making it difficult for prowling animals and heavy winds to damage them. Unfortunately, it also can make harvesting for us humans a little more difficult as well.
I planted my three sisters at the same time this year because I was in a hurry. It looks like the beans are "loving the corn stalks to death" and may even pull them down if I don't add some stakes. Next year, I'll be sure to go back to the original plan and let the corn get two or three feet tall before planting the beans. The squash always goes in at the same time as the corn. I plant mine in 4-foot-wide circles so I can reach my short arms into the center from all sides to harvest.
Next year I may try a fun experiment and plant a very tall variety of corn in a 6 foot circle. As the corn grows, I'll gently bend the stalks toward a pole positioned in the center of the circle, creating a tee-pee for my grandchildren to play in. I'll plant the squash and beans at the outer edge of the corn circle so the corn will still have her sisters to "play" with when the kids are away. Experimenting with new ideas is part of the fun of gardening, so why not have a little fun?