  We study a class of leptogenesis models where the light neutrinos acquire
their observed small masses by a symmetry-motivated construction. This class of
models may naturally include three nearly degenerate heavy Majorana neutrinos
that can strongly mix with one another and have mass differences comparable to
their decay widths. We find that such a tri-resonant heavy neutrino system can
lead to leptonic CP asymmetries which are further enhanced than those obtained
in the usual bi-resonant approximation. Moreover, we solve the Boltzmann
equations by paying special attention to the temperature dependence of the
relativistic degrees of freedom of the plasma. The latter results in
significant corrections to the evolution equations for the heavy neutrinos and
the lepton asymmetry that have been previously ignored in the literature. We
show the importance of these corrections to accurately describe the dynamical
evolution of the baryon-to-photon ratio $\eta_B$ for heavy neutrino masses at
and below $100$ GeV, and demonstrate that successful leptogenesis at lower
masses can be significantly affected by the variation of the relativistic dofs.
The parameter space for the leptogenesis model is discussed, and it could be
probed in future experimental facilities searching for charged lepton flavour
violation and heavy neutrinos in future $Z$-boson factories.
