For the Fe-Zr-Nb system, ion beam mixing of multiple metal layers with various compositions was carried out. Using high resolution transmission electron microscopy analyses, nine compositions were determined to be favored for metallic glass formation . It is found that all these composition points fall in a hexagonal region defined by linearly connecting the six composition points determining the glass-forming ranges of its sub-binary system. It is therefore proposed to predict the favored glass-forming region for a ternary metal system by the glass-forming ranger of its sub-binary system.