The three modes of heat transfer are conduction, convection, and radiation. Here is a brief explanation of each mode:

1. Conduction: This mode of heat transfer occurs when heat is transferred from one part of a substance to another part without the movement of the molecules of the substance. The rate of heat transfer depends on the temperature gradient.

2. Convection: This mode of heat transfer occurs when the transfer of heat takes place from one part to another part of the substance or fluid due to the actual motion of the molecules. The rate of heat transfer depends mainly on the difference in the fluid density at different temperatures.

3. Radiation: This mode of heat transfer occurs when the heat transfer takes place from the source of heat to the substance to be heated without direct contact between them. The heat transfer through radiation depends upon the surface.

In most real situations, heat transfer occurs as a result of a combination of these modes of heat transfer.