  We clarify how mirror symmetry acts on 3d theories with N=2,3 or 4
supersymmetries and non-abelian Chern-Simons terms and then construct many new
examples. We identify a new duality, geometric duality, that allows us to
generate large families of gauge theories, with and without Chern-Simons term,
that all flow to the same conformal field theory in the infrared. In
particular, we find an interesting duality of dualities: a pair of theories
related via mirror symmetry can be mapped, via geometric duality, into a pair
of gauge theories related by Seiberg duality. This network of dualities can be
understood as the simple result that all of these theories are different
realizations of one and the same system in M-theory.
