The three parts of the Earth are Atmosphere, Hydrosphere and Lithosphere.
Atmosphere is the blanket of air that surrounds the earth. It is densest close to the surface and thins out as one moves higher. Atmosphere of Earth contains mainly Nitrogen, followed by Oxygen and small amounts of water vapor, Carbondioxide and other gases.
Lithosphere is the outer most part of the earth's surface. The Earth's crust and the mantle form Lithosphere.
Hydrosphere is the part of the Earth that has water. The Oceans, seas, rivers, lakes and other water bodies constitute the Hydrosphere.
Stratosphere, Mesosphere and Ionosphere are different layers of atmosphere.
Hence, for the study of the Earth, one needs to consider earth to be made of three parts- atmosphere, Lithosphere and Hydrosphere.
