The present study investigates three-dimensional gap resonance between two fixed side-by-side barges under combined wave and uniform current excitation using a fully nonlinear numerical wave tank based on the Navier–Stokes equations. It examines how currents aligned with regular waves affect the gap response under head and beam seas. In beam seas, the free surface in the gap primarily exhibits a modal-type response in the form of a standing wave. The maximum gap response, consistently occurring at the midpoint of the gap, increases gradually with the current speed. Conversely, in head seas, the maximum response decreases slightly with increasing current speed, and the occurring location shifts downstream. Moreover, resonant free-surface responses along the gap in head seas manifest as propagating waves rather than modal-type standing waves, consisting of a wider spectrum of wave components around the resonant ones and traveling faster than the incident waves regardless of current speed. The wavelengths of those resonant waves tend to increase with increasing current speed. Additionally, the presence of current significantly enlarges the transverse first-harmonic and mean-drift wave forces on the barges under beam-sea conditions. The study highlights the necessity of considering current effects on three-dimensional gap resonances in marine operations at coastal and offshore locations.