  The effective potential has been previously calculated through three-loop
order, in Landau gauge, for a general renormalizable theory using dimensional
regularization. However, dimensional regularization is not appropriate for
softly broken supersymmetric gauge theories, because it explicitly violates
supersymmetry. In this paper, I obtain the three-loop effective potential using
a supersymmetric regulator based on dimensional reduction. Checks follow from
the vanishing of the effective potential in examples with supersymmetric vacua,
and from renormalization scale invariance in examples for which supersymmetry
is broken, either spontaneously or explicitly by soft terms. As byproducts, I
obtain the three-loop Landau gauge anomalous dimension for the scalar component
of a chiral supermultiplet, and the beta function for the field-independent
vacuum energy.
