A refrigerator works on the principle that heat is absorbed by a gas as it expands and when the gas is compressed heat is released by the gas. A gas under a high pressure or liquid coolant flows through the interior of the refrigerator; it absorbs heat and is converted to a low pressure gas. The gas then passes through a compressor that compresses it back to liquid. This results in a lot of heat being released and the temperature of the coolant increasing. The hot liquid is passed through a set of pipes placed outside the refrigeration unit and heat is dissipated to the surrounding air.
The work done to increase the volume of a gas at a constant pressure is given by W = P*V. Here the pressure in 3 atm. or 3*101.325 * 10^3 Pa and the volume is 3 liters or 3*10^-3 m^3.
W = 3*101.325*10^3*3*10^-3 J = 911.9 J.
The heat absorbed is equal to the work. 911.9 J of heat is absorbed.