Following Part I [X. Yao, et al., J. Mater. Res. 23(5), 1282 (2008)] and Part 11 [X. Yao, et al., J. Mater. Res. 23(5), 1292 (2008)] the cellular automation-finite control volume method (CAFVM) model was used to study the grain formation and microstructure morphology resulting from solidification of a commercial Al-Si-Mg alloy with Al-Ti-B grain refiner additions. The model incorporates the effect of the introduced solute Ti and the alloying elements of Si and Mg on the growth restriction factor, constitutional undercooling, and nucleation parameters. With respect to grain refinement, it is found that the alloying elements, Si and Mg, play a role that is similar to Ti qualitatively while different quantitatively. Accordingly, a concept of "equivalent solute" determined by phase diagram parameters such as the solute partitioning coefficient and the liquidus slope is proposed to clarify the effect of each solute in the alloy on grain formation during solidification. Based on the calculations and on comparison to the experimental data, a possible mechanism of grain refinement in this alloy system is proposed.