  We describe the procedure of the construction of a sample of distant
($z>0.3$) radio galaxies using the NED, CATS and SDSS databases for further use
in various statistical tests. We believe the sample to be free of objects with
quasar properties. This paper is the third part of the description of the radio
galaxies catalog that we plan to use for cosmological tests. We report the
results of the sample of angular sizes for the NVSS survey list objects, and
its preliminary statistical analysis. Three-parameter diagrams "angular
size-redshift-flux density" and "angular size-redshift-spectral index", and
their two-parameter projections are constructed. Three subsamples of radio
galaxies are separated in the "source size--flux density" diagram.
