  The results of 3-4 $\mu$m spectroscopy towards the nuclei of NGC 3094, NGC
7172, and NGC 7479 are reported. In ground-based 8-13 $\mu$m spectra, all the
sources have strong absorption-like features at $\sim$10 $\mu$m, but they do
not have detectable polycyclic aromatic hydrocarbon (PAH) emission features.
The 3.4 $\mu$m carbonaceous dust absorption features are detected towards all
nuclei. NGC 3094 shows a detectable 3.3 $\mu$m PAH emission feature, while NGC
7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption
features but no PAH emission features is thought to dominated by highly
obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and
three other such nuclei in the literature, we investigate the optical depth
ratios between the 3.4 $\mu$m carbonaceous dust and 9.7 $\mu$m silicate dust
absorption ($\tau_{3.4}$/$\tau_{9.7}$). The $\tau_{3.4}$/$\tau_{9.7}$ ratios
towards three highly obscured AGNs with face-on host galaxies are
systematically larger than the ratios in the Galactic diffuse interstellar
medium or the ratios for two highly obscured AGNs with edge-on host galaxies.
We suggest that the larger ratios can be explained if the obscuring dust is so
close to the central AGNs that a temperature gradient occurs in it. If this
idea is correct, our results may provide spectroscopic evidence for the
presence of the putative ``dusty tori'' in the close vicinity of AGNs.
