A method for calculating saturation magnetostriction values in solid-solution ternary alloys is proposed using the magnetostriction data of binary alloys implicated in the ternary system and few data of ternary alloy. Studies on three systems are presented: the Fe-Al-Si system where a considerable magnetostriction data available in the literature was employed to perform the method; the Fe-Al-Ge system in which our data are compared and discussed with a previous study in this system; and the Fe-Ge-Si system for which iso-magnetostriction curves are proposed since only one experimental value in the literature was found. The results obtained by this method approximate previously reported experimental data. This can be used to design alloys with a defined value of magnetostriction reducing the number of samples and measurements or in calculations on theoretical models. In this sense, a hypothetical behaviour of magnetostriction in the Fe-Ge binary system for Ge > 20 at.%. is also presented.