Ni-Sn-Zr alloys have many interests in different fields of application going from nuclear material applications to functional materials (such as thermoelectrics). This wide application range requires a good knowledge of the stability of the phases. In the present study, first principles calculations are used to determine the enthalpies of formation of intermediate phases and of the structural defects in some structures. Based on these data and on the literature, the stability of the phases is discussed at low and high temperatures. Finally, for the first time the complete ternary system is described in the whole temperature range.