The search of minimal enthalpy structures of ternary magnesium alloys of different stoichiometry MgxSiyGez under pressure P ≤ 6 GPa has been performed using the software suite USPEX implementing the evolution algorithm combined with the density functional theory (DFT) approach. The evolutionary search has yielded new possible ternary compounds of the stoichiometries Mg12Ge3Si, Mg6Ge4Si3, Mg6Ge3Si4, Mg6GeSi3, Mg4Ge3Si2, Mg4Ge3Si2, Mg4SiGe, and Mg2GeSi, which have negative enthalpy of formation at pressures in the range of 0 to 10 GPa and which are not substitution solutions of Ge on Mg2Si matrix (or Si on Mg2Ge matrix). These compounds have metallic properties and formation enthalpies comparable to those of binary silicides Mg2Si. Phonon spectra investigations indicate that only first five of them are stable showing no imaginary frequencies.