The phase diagram of the pseudoternary (Hf,Ti,Zr)NiSn system is of interest for the selection of the compositions and for the prediction of the stability of the final material prior to measuring its thermoelectric properties. However, a review of the experimental data shows that it is difficult to obtain stable and acceptable alloys. To overcome this difficulty we have performed ab-initio calculations to predict the phase separations in the half Heusler phase. Our calculations are consistent with the experimental data and we obtain the evolution of the solid-solution from 500 to 1300 K the temperature range of interest of these materials.