This paper documents the breakdown of gravity waves generated by deep convection in a three-dimensional cloud-resolving model. The convection generates gravity waves that propagate into the lower stratosphere, with horizontal wavelengths between 5 and 10 km. Above-cloud wind shear causes part of the spectrum of these waves to break, inducing overturning. The model grid spacing is small enough (150 m) that the gravity waves are well resolved, and the turbulent cascade induced by the breakdown is partially resolved. Previous model simulations of wave breakdown above deep convection, at this resolution, have only been achieved in two-dimensional models. The wave breakdown generates secondary waves, which have much shorter horizontal wavelengths, and different propagation characteristics compared to the primary waves. Secondary wave generation in the lower stratosphere above deep convection has not been identified in previous studies. The wave breakdown also induces irreversible mixing, which is quantified in terms of the vertical transport of water vapor.