We investigate a three-waveguide tapered velocity coupler which consists of a uniform linear, tapered linear and tapered nonlinear slab waveguide. The coupling characteristics depending on the gap width between the waveguides and sloping angle of the tapered waveguides are analyzed by means of the finite difference method. The numerical results show that with realistic structural parameters flat output power characteristics can be obtained over a wide range of input power. It is found that it is possible to use the present structure as a power limiter.