  In this paper we investigate three-dimensional superconformal gauge theories
with N=3 supersymmetry. Independently from specific models, we derive the
shortening conditions for unitary representations of the Osp(3|4) superalgebra
and we express them in terms of differential constraints on three dimensional
N=3 superfields. We find a ring structure underlying these short
representations, which is just the direct generalization of the chiral ring
structure of N=2 theories. When the superconformal field theory is realized on
the world-volume of an M2-brane such superfield ring is the counterpart of the
ring defined by the algebraic geometry of the 8-dimensional cone transverse to
the brane. This and other arguments identify the N=3 superconformal field
theory dual to M-theory compactified on AdS_4 x N^{0,1,0}. It is an N=3 gauge
theory with SU(N) x SU(N) gauge group coupled to a suitable set of
hypermultiplets, with an additional Chern Simons interaction. The AdS/CFT
correspondence can be directly verified using the recently worked out Kaluza
Klein spectrum of N^{0,1,0} and we find a perfect match. We also note that
besides the usual set of BPS conformal operators dual to the lightest KK
states, we find that the composite operators corresponding to certain massive
KK modes are organized into a massive spin 3/2 N=3 multiplet that might be
identified with the super-Higgs multiplet of a spontaneously broken N=4 theory.
We investigate this intriguing and inspiring feature in a separate paper.
