Explain the differences between plains, plateaus mountains

Plateau - land area having a relatively level surface considerably raised above adjoining land on at least one side, and often cut by deep canyons

plain is a flat area. Plains occur as lowlands along the bottoms of valleys, coastal plains and as plateaus or uplands at high elevations. In a valley, a plain is enclosed on two sides but in other cases a plain may be delineated by a complete or partial ring of hills, by mountains or cliffs

mountains a natural elevation of the earth's surface rising more or less abruptly to a summit, and attaining an altitude greater than that of a hill, usually greater than 2000 ft.