We investigate the third-order spontaneous parametric down-conversion process in a
nonlinear media with inversion centers. Specifically, we analyze in details the three-photon
differential count rate in unit frequency and angular regions, total count rate and
measurement time for rutile and calcite crystals which have comparatively large cubic
susceptibilities. Special attention is given to consideration of limited frequency and angular
detection ranges in order to calculate experimentally available detection rate values.