The complete literature review and critical thermodynamic optimization of the Mg–Ge–Si, Mg–Ge–Sn, Mg–Pb–Si and Mg–Pb–Sn systems have been performed in the present study. The thermodynamic behavior of liquid solution was described by the Modified Quasichemical Model, which takes into account the short-range-ordering tendency of first-nearest-neighbor atoms. The Mg2X solid solution (X = Ge, Pb, Si, Sn) was modeled with the assumption of random mixing of X atoms in the solution. As results of the present thermodynamic modeling, the experimentally unexplored ternary phase diagrams of the Mg–Ge–Si, Mg–Ge–Sn and Mg–Pb–Si systems were predicted for the first time. The variation of mixing enthalpy of liquid phase can be accurately explained by the Modified Quasichemical Model.