Structural studies of the layered superionic conductors Li3N and Li3P and the semimetallic conductor Li3As are reported. X-ray diffraction measurements show that these compounds crystallize in a hexagonal structure with a P6/mmm space group. The lattice dimensions expand anisotropically as the size of the anion increases. The temperature dependence of the Fourier transform infrared (FTIR) absorption spectra of Li3X compounds have been studied in the spectral range 20-700 cm-1. The complex dielectric response was analyzed using Lorentzian-oscillator fits. The frequency and damping of the infrared-active phonons are determined as well as the permittivity constants. These data illustrate the change in lattice dynamics in Li3X compounds. The deduced infrared effective charge and electronic polarizability indicate the chemical bonding modification for the Li3X compounds. The relationship between polarizability and conduction is addressed.