  We use brane configurations and SL(2,Z) symmetry of the type IIB string to
construct mirror N=2 supersymmetric gauge theories in three dimensions. The
mirror map exchanges Higgs and Coulomb branches, Fayet-Iliopoulos and mass
parameters and U(1)R symmetries. Some quantities that are determined at the
quantum level in one theory are determined at the classical level of the
mirror. One such example is the complex structure of the Coulomb branch of one
theory which is determined quantum mechanically. It is mapped to the complex
structure of the Higgs branch of the mirror theory, which is determined
classically. We study the generation of N=2 superpotentials by open D-string
instantons in the brane configurations.
