About two thirds of the world's human population survive primarily by eating one or more of three grain crops. what are these three grains?

The answer is rice, wheat, and corn.

Croplands, rangelands and pastures, and fisheries and aquaculture provide an important supply of world's food. Nearly 80% of the world's food comes from croplands. People can eat about 50,000 plant species, but only 14 of them supply the majority of world's food. Of those 14, only three grains, and those are rice, wheat, and corn, are consumed by about two-thirds of the world's human population.