How does thermal energy affect the three states of matter?

An increase in thermal energy, changes the state of matter from solid to liquid to gas.

The thermal energy of a system affects the motion of molecules. One of the postulates of kinetic molecular theory states that, the average speed of molecules is directly proportional to the absolute temperature. If you increase the thermal energy of a system, a solid for example will turn into liquid (it will melt). A solid is rigid because of the strong intermolecular force holding the molecules together. If thermal energy is increased, this energy will be converted to kinetic energy and will make the molecules to move fast. If they move fast enough, they would overcome the intermolecular force holding them, thus making the solid molecules glide from one another and become a liquid.