  We study the 3D field theory on one D3-brane stretched between (r,s) and
(p,q)5-branes. The boundary conditions are determined from the analysis of NS5
and D5 charges of the two 5-branes. We carry out the mode expansions for all
the fields and identify the field theory as Maxwell-Chern-Simons theory. We
examine the mass spectrum to determine the conditions for unbroken
supersymmetry (SUSY) in this field theory and compare the results with those
from the brane configurations. The spectrum is found to be invariant under the
Type IIB SL(2,{\bf Z})-transformation. We also discuss the theory with matters
and its S-dual configuration. The result suggests that the equivalence under
S-duality may be valid if we include all the higher modes in the theories with
matters. We also find an interesting phenomenon that SUSY enhancement happens
in the field theory after dimensional reduction from 3D to 2D.
