In this study, the discharge performance of the sluice passageway of tidal power plants was investigated based on the experiments conducted in a planar open channel and three-dimensional numerical simulations. By conducting the experiments in a planar channel, it was possible to reproduce the three-dimensional flow field around the sluice passageways similar to the field condition. The discharge capability of the passageway was estimated under various flow conditions with five different channel bathymetries. The estimates of the discharge coefficient generally ranged from 1.3 to 1.45, which are significantly smaller than the values obtained from the previous study based on the two-dimensional experiment. In addition, the experimental results showed a considerable difference in the discharge coefficient among the test cases, demonstrating an apparent influence of channel bed topography on the discharge performance. Based on an intensive parametric study carried out using the numerical simulations, an optimal configuration of the width, slope, and bottom length of the apron section was suggested for maximizing the discharge capability of the sluice passageway.