  Gauge-coupling unification is just as successful in the standard model with
six Higgs doublets as it is in the minimal supersymmetric standard model.
However, the gauge couplings unify at 10^14 GeV, which yields rapid proton
decay in the SU(5) model. I propose that the grand-unified gauge group is
instead SU(3)_c x SU(3)_L x SU(3)_R, in which baryon number is conserved by the
gauge interactions.
