  We show how a recent proposal to obtain the distribution of conductances in
three dimensions (3D) from a generalized Fokker-Planck equation for the joint
probability distribution of the transmission eigenvalues can be implemented for
all strengths of disorder by numerically evaluating certain correlations of
transfer matrices. We then use this method to obtain analytically, for the
first time, the 3D conductance distribution in the insulating regime and
provide a simple understanding of why it differs qualitatively from the
log-normal distribution of a quasi one-dimensional wire.
