We show that the four derivative terms in the effective action of three-dimensional N=8 Yang-Mills theory are determined by supersymmetry. These terms receive both perturbative and non-perturbative corrections. Using our technique for constraining the effective action, we are able to determine the exact form of the eight fermion terms in the supersymmetric completion of the F 4 term, including all instanton corrections. As a consequence, we argue that the integral of the Euler density over k monopole moduli space in SU(2) Yang-Mills is determined by our non-renormalization theorem for all values of k.