The Monte Carlo simulation code ACAT-DIFFUSE-GAS has been developed in order to simulate the whole system of a planar magnetron sputtering discharge. The atomic collisions in the Tb-Fe-Co alloy cathode, which lead to sputtering and reflection, are simulated by the ACAT-DIFFUSE routine which includes the compositional change induced by ion influence. The thermalization of sputtered Tb, Fe and Co atoms and reflected Ar atoms in the background Ar gas within the planar magnetron sputtering discharge are simulated by the GAS routine, using the Monte Carlo technique. It is found that the deposited composition rate of Tb, Fe and Co depends strongly on the background pressure. The appreciable amount of energetic reflected Ar atoms transmit at the low pressure and deposit their energies on the magneto-optical him.