By calorimetry method solubility enthalpies of binary alloys of Tb–Mg system and ternary alloys of Cerium (Prasiodym, Therbium) with Magnesium and Indium of the group at standard conditions (298 K) have been determined and enthalpies of their formation have been calculated. In binary Tb–Mg system the compound of TbMg (78 ± 3 kJ/(mole · atom)) possesses the greatest absolute enthalpy value. For ternary alloys the influence of the third component addition on the stability of alloys has been shown. Ternary alloys possess higher absolute value of formation enthalpies. The formation of the most stable binary compounds and solid solutions hardens thermodynamical stability of alloys. Increase in REM content leads to non-monotonic change in alloys formation enthalpies values. In Ce–Mg– In, In–Mg–Pr and Tb–In–Mg systems sufficiently high absolute enthalpy formation values are characteristic of the alloys containing 18.22 at.% Сe and 19.92 ат.% Mg (92.08 ± 0.05 kJ/(mole · atom), 54.24 at.% Mg, and 36.61at.% Pr (54.12 kJ/(mole · atom) and 7.50 ± 0,08 at.% Mg, 74.63 ± 0.14 at.% Tb (146.1 ± 0.2 kJ/(mole · atom)) respectively.