By means of band calculations within the FLAPW (full-potential linearized augmented-plane-waves) model, the role of mutual atomic substitutions in the formation of energy, charge, and spin characteristics of $M$MnSb alloys ($M$ = Co, Ni, Cu) is clarified. As found, their ground state (with the largest cohesive energy) is realized in the case of the arrangement of $M$ atoms at the crystallographic positions with tetrahedral atomic environment, and their substitution by the manganese or antimony atoms leads to the appearance of metastable phases with high energy-excited state. The exchange of atoms between the positions, which forms the NaCl-type sublattice, does not lead to changes in the energy characteristics and electronic structure of alloys. The nickel- and cobalt-containing alloys in the ground state are semimetals, while the alloys with copper and alloys in a metastable state are metals. For all the alloys, the largest magnetic moment is localized on the manganese atoms.