General expression for three-loop $\beta$-functions of $\cal N =$ 1 supersymmetric theories with multiple gauge couplings regularized by higher covariant derivatives

For $\cal N =$ 1 supersymmetric theories with multiple gauge couplings regularized by higher covariant derivatives, a general expression for three-loop gauge $\beta$-functions is obtained. For this purpose, using general statements about the validity of the NSVZ relations, a result is constructed for the $\beta$-functions defined in terms of the bare couplings. It is demonstrated that in the particular case of MSSM, it precisely reproduces the known expressions found earlier in another way. For the case where Yukawa couplings are absent, similar expressions for three-loop $\beta$-functions are also obtained in terms of the renormalized couplings in an arbitrary subtraction scheme and, in particular, in the $\overline{\mbox{DR}}$ scheme.