1. conduction
Heat is transferred when the particles in the hotter side vibrate and collide with the neighbouring particles. The rate of heat transfer is the highest in solid, followed by liquid and then gas. It is because the particles in solid are closely packed together, increasing the frequency of collisions. Heat cannot be transferred in vacuum by conduction because there are no particles.
2. convection
It only applies to fluid, ie gas and liquid. Heat can’t be transferred in solid and vacuum by convection. When the liquid is heated, the hotter part expands in volume, hence lowering its pressure. This part then rises and forces the colder part to sink. This forms a convection current.
3. radiation
This can occur in any situation, including vacuum. All objects emits and absorbs infrared radiation. A hotter object has a higher emission rate than the absorption rate
