  Three-loop $\beta$-functions of the Minimal Supersymmetric Standard Model
regularized by higher covariant derivatives are obtained for an arbitrary
supersymmetric subtraction scheme. For this purpose we first calculate two-loop
anomalous dimensions for all MSSM chiral matter superfields defined in terms of
the bare couplings. Then we use the NSVZ equations for the renormalization
group functions defined in terms of the bare couplings, which are valid in all
orders in the case of using the higher covariant derivative regularization.
This gives the three-loop $\beta$-functions defined in terms of the bare
couplings. After that, we construct the three-loop $\beta$-functions and the
two-loop anomalous dimensions standardly defined in terms of the renormalized
couplings for an arbitrary subtraction scheme. As a nontrivial correctness
test, we verify that for a certain renormalization prescription the general
results reproduce the ones obtained earlier in the $\overline{\mbox{DR}}$
scheme. Also this can be considered as an indepedent confirmation of the
$\overline{\mbox{DR}}$ results.
