  We present an X-ray study of three mixed-morphology supernova remnants
(SNRs), HB 21, CTB 1 and HB 3, using archival ASCA and ROSAT data. These data
are complemented by archival Chandra X-ray Observatory data for CTB 1 and
XMM-Newton X-ray Observatory data for HB 3. The spectra from HB 21 and HB 3 are
well-described with a single-temperature thermal plasma in ionization
equilibrium, while a two-temperature thermal plasma is found in CTB 1. We found
enhanced abundances in all three SNRs. The elemental abundance of Mg is clearly
enhanced in CTB 1, while HB 21 has enhanced abundances of Si and S. The
situation is not so clear in HB 3 -- the plasma in this SNR either has
significantly enhanced abundances of O, Ne and Mg, or it has marginally
enhanced abundances of Mg and under-abundant Fe. We discuss the plausibility of
mixed-morphology SNR models for the three SNRs and the presence of enhanced
abundances. We revise a list of MM SNRs and their properties, compare the three
SNRs studied here with other members of this class, and discuss the presence of
enhanced elemental abundances in MM SNRs. We also report the ASCA detection of
a compact source in the southern part of HB 3. The source spectrum is
consistent with a power law with a photon index of ~2.7, and an unabsorbed
X-ray flux of ~10^{-12} erg/cm^2/s in the 0.5--10.0 keV band. The column
density towards this source differs from that towards the SNR, and it is
therefore unlikely they are related.
