  We have compiled a sample of 234 ultra-steep-spectrum(USS)-selected radio
sources in order to find high-redshift radio galaxies (HzRGs). The sample is in
the southern sky at -40 deg < DEC < -30 deg which is the overlap region of the
408-MHz Revised Molonglo Reference Catalogue, 843-MHz Sydney University
Molonglo Sky Survey (the MRCR--SUMSS sample) and the 1400-MHz NRAO VLA Sky
Survey. This is the third in a series of papers on the MRCR--SUMSS sample. Here
we present optical spectra from the ANU 2.3-m telescope, ESO New Technology
Telescope and ESO Very Large Telescope for 52 of the identifications from
Bryant et al. (2009, Paper II), yielding redshifts for 36 galaxies, 13 of which
have z>2. We analyse the K-z distribution and compare 4-arcsec-aperture
magnitudes with 64-kpc aperture magnitudes in several surveys from the
literature; the MRCR--SUMSS sample is found to be consistent with models for
10^{11}-10^{12} solar mass galaxies. Dispersions about the fits in the K-z plot
support passive evolution of radio galaxy hosts since z>3. By comparing
USS-selected samples in the literature, we find that the resultant median
redshift of the samples shown is not dependent on the flux density distribution
or selection frequency of each sample. In addition, our finding that the
majority of the radio spectral energy distributions remain straight over a wide
frequency range suggests that a k-correction is not responsible for the success
of USS-selection in identifying high redshift radio galaxies and therefore the
steep radio spectra may be intrinsic to the source or a product of the
environment. Two galaxies have been found to have both compact radio structures
and strong self-absorption in the Ly-alpha line, suggesting they are surrounded
by a dense medium...abridged.
