  The phase diagram of the half-filled spinless Holstein model for electrons
interacting with quantum phonons is derived in three dimensions extending at
finite temperature $T$ a variational approach introduced for the
one-dimensional T=0 case. Employing the variational scheme, the spectral and
optical properties of the system are evaluated in the different regimes that
characterize the normal and ordered state. The effects of the charge-ordering
($CO$) induce a transfer of spectral weight from low to high energies in the
conductivity spectra, as the temperature decreases or the strength of the
electron-phonon ($el-ph$) interaction increases. The inclusion of effects of
lattice fluctuations is able to smooth the inverse square-root singularity
expected for the case of the mean-field approach and determines a subgap tail
absorption. Moreover, in the weak to intermediate coupling regime, a
two-component structure is obtained within the $CO$ phase at low frequency: the
remnant Drude-like term and the incipient absorption band centered around the
gap energy.
