This paper is the third part of a theory of the coherent propagation of light in semiconductors. Based on the previous papers (Ⅰ) and (Ⅱ), the Maxwell-Bloch equation describing the coherent propagation of light in semiconductors is derived; the equation is then used to discuss a number of possible effects of coherent propagation in both interband transitions and exciton transitions. In particular, the phenomena of self-induced transparency are analysed, and it is shown that the phenomena may occur in the above two transitions.