[en] The heat capacities of Nb₃Al, Nb₃Sn, and Nb₃Sb were measured between 2 and 400 K. Their superconducting transition temperatures T/subc/ are 18.5, 17.9, and 0.2 K, respectively. The higher-temperature entropy was analyzed, and it was found that phonon-mode softening occurs on cooling for the two high-T/subc/ compounds but not for Nb₃Sb. Geometric mean phonon frequencies ω/subg/ are also evaluated for these compounds and for V₃X, where X = Si, Ga, Ga₀.₅Sn₀.₅, and Sn. As is shown, ω/subg/ is an appropriate phonon average for evaluating McMillan's expression for the electron-phonon mass enhancement. Average phonon properties cannot reliably be used to calculate T/subc/ values for these A-15 materials. It is suggested that, for the higher electronic density-of-states materials, select phonon modes strongly couple to the electronic system and influence the magnitude of T/subc/ to a greater extent than average-phonon correlations would indicate. This view is supported by the observation that the higher-density-of-states materials exhibit the more pronounced phonon-mode softening on cooling