We renormalize to three loops a version of the Thirring model where the
fermion fields not only lie in the fundamental representation of a non-abelian
colour group SU(N_c) but also depend on the number of flavours, N_f. The model
is not multiplicatively renormalizable in dimensional regularization due to the
generation of evanescent operators which emerge at each loop order. Their
effect in the construction of the true wave function, mass and coupling
constant renormalization constants is handled by considering the projection
technique to a new order. Having constructed the MSbar renormalization group
functions we consider other massless independent renormalization schemes to
ensure that the renormalization is consistent with the equivalence of the
non-abelian Thirring model with other models with a four-fermi interaction. One
feature to emerge from the computation is the establishment of the fact that
the SU(N_f) Gross Neveu model is not multiplicatively renormalizable in
dimensional regularization. An evanescent operator arises first at three loops
and we determine its associated renormalization constant explicitly.Comment: 40 latex pages, 14 postscript figure
