  We study the thermoelectric transport properties of a three-dimensional
massive relativistic fermion gas with screened Coulomb impurities in high
magnetic fields where only the lowest Landau levels contribute to the
transport. Our results can be applied to experimental results of gapless and
gapped Dirac materials. We focus on the effects of the mass term and we show
the main differences that arise compared to the massless Dirac fermions. The
different behavior is shown to be relevant at higher magnetic fields. The
calculations are performed in the framework of the linear response theory using
the exact quantum mechanical solution of the system in a constant magnetic
field. We prove that the Mott formula and the Wiedemann-Franz law are valid at
low temperatures and use them to calculate the thermoelectric transport
coefficients. We show that the temperature range where the low temperature
approximation is valid increases with increasing magnetic fields. The magnetic
field dependence of measurable quantities (i.e. conductivity, Seebeck
coefficient, Nernst coefficient and thermal conductivity) strongly depend on
the magnetic field dependence of the scattering rate, thus the result relies on
the proper treatment of the impurities. In this work they are included through
the first Born approximation using screened charged impurities as impurity
potential. We show that the electric conductivity does not change qualitatively
in the case of finite mass term. On the other hand we find that the mass term
causes significantly different behavior in the Seebeck and Nernst coefficients.
