Nonempirical study. of the site preference occupation for Co and Mn substituting for Fe in Fe3Al has been carried out for a number of Co and Mn concentrations in the framework of the coherent potential approximation. Obtained values of total energies show in a full agreement with experiments that Mn atoms in the equilibrium configuration occupy the same iron sublattice from 0.5% up to 5% of Mn in the Fe3Al-based alloy. Calculations of alloys with the Co-doped iron sublattices of the D0(3) phase show completely different character of bonding between different concentrations of Co alloying. These feature leads to the different tendencies in occupation of Fe sublattices.