  We calculate the three loop gauge $\beta$-function for an abelian $N=1$
supersymmetric gauge theory, using DRED. We construct a coupling constant
redefinition that relates the result to the corresponding term in the NSVZ
$\beta$-function, and by generalising this redefinition to the non-abelian case
we derive the DRED three loop gauge $\beta$-function for the non-abelian case.
