The generation of narrow-band radiation in resonant cavities due to the transit of a high energy electron bunch is shown numerically in three dimensions. Previous studies only examined this in two dimensions (Robinson 2025 Plasma Phys. Control. Fusion67 045026), however there appears to be no case for this not equally working in three dimensions. The finite bandwidth of the radiation when a loss-inducing exit channel is included was also examined, and bandwidths as low as a few percent are demonstrated.