In this Part 3 of the series, we discuss work done on the rates of complex fluid-solid reactions. It is shown that many of the complex systems can be analyzed by recognizing the component single fluid-solid reactions. The overall reaction system is then quantitatively described by combining the mathematical relationships for the component single reactions discussed in Parts 1 and 2.([1,2]) In many cases, Sohn's law of additive reaction times discussed in Part 2 is shown to provide a useful means of simplifying the quantitative description of the complex fluid-solid reactions.([2])