One of the major aims of lattice QCD (LQCD) is to calculate electroweak decay and transition amplitudes from first principles, so as to use experimental results to test the standard model. A classic example is the $K \to \pi\pi$ amplitude, and in particular its CP-violating part, where experimental results have long been available but which only recently has been accessible to LQCD. In this talk I describe the status of efforts to extend the methodology to processes involving three or more particles, e.g. $K\to 3\pi$, one of whose aims is to eventually calculate CP violation in D decays in the standard model. This requires also understanding how to use LQCD to determine the properties of hadronic resonances that couple to three or more particles.