The effects of variations in middle-ear cavity size on hearing sensitivity in the Mongolian gerbil are predicted by computing the effect on acoustic power input to the middle ear. Acoustic power collection from a diffuse sound field was computed from measurements of the middle-ear input impedance and external-ear radiation impedance and mathematical models of the middle and external ear presented in the first two papers of this series [J. Acoust. Soc. Am. 92, 157-177 (1992); J. Acoust. Soc. Am. 99, 3044-3063 (1996)]. A reduction in middle-ear cavity volume to 1/4 its normal value is predicted to cause a frequency-selective elevation in auditory threshold of as much as 12 dB, with the largest elevation occurring in the 1-2 kHz range. Greater reductions produce larger threshold elevations. Increases in cavity volume cause decreases in the predicted threshold of at most 12 dB. Threshold predictions for volumes equal to those of the smaller hamster and the larger kangaroo-rat middle-ear cavity volumes resemble threshold functions measured in those animals. Results are consistent with the idea that large middle-ear cavities evolved in gerbil to improve hearing sensitivity below 3 kHz and thereby improve the animal's chances for survival.