The experimental and theoretical study of a family of linear trimeric [NEt4] {[Mn(L)]2[M(CN)6]} (M = Fe, Ru,Os) compounds is reported. All three complexes demonstrate slow relaxation of magnetization at low temperatures. The theoretical model for the explanation of the magnetic behavior (the magnetic susceptibility as a function of temperature and magnetization as a function of the applied magnetic field) includes the spin-orbital interaction acting within the ground 2T1 multiplet of the FeIII, RuIII or OsIII ion, the axial crystal field that splits this multiplet into an orbital singlet and orbital doublet, the zero-field splitting for the MnIII ions, the isotropic exchange interaction in the M-Mn pairs as well as the intercluster interaction taken within the mean field approximation. The proposed model provides a good agreement between the observed and calculated magnetic behavior. The low-lying energy levels form the energy barriers for magnetization reversal for all examined compounds. The calculated heights of these barriers are close to the observed ones.