  We calculate instanton corrections to three dimensional gauge theories with
N=4 and N=8 supersymmetry and SU(n) gauge groups. The N=4 results give new
information about the moduli space of n BPS SU(2) monopoles, including the
leading order non-pairwise interaction terms. A few comments are made on the
relationship of the N=8 results to membrane scattering in matrix theory.
