Corn does grow from a grass, quite a small one called teosinte. It is a small grass that produces hard-to-get-at, TINY grains and was genetically cultured into the grain we know today.
Most of the peoples in MesoAmerica and North American treated their corn grains, however. Even today, a real tortilla is made by first soaking the corn in a lime (the stone, not the fruit) solution to break down the anti-nutrients and free up the good stuff to make it more edible. The corn is then drained, ground into a dough and made into the thin flatbreads that are then cooked over a hot fire on a stone surface.
Also, as to the three sisters, the squash was very important because it provided the greens! The leaves and flowers were eaten just as much, if not more so than the fruit itself. The fruits were of course often used to make dishes and other container-type tools. Beans were often treated the same was as corn, soaking and fermenting them. Sprouting certainly occurred as well. Corn was also used as the basis of some fermented drinks, though most in the South came from the agave instead (pulque is my favorite).