Based on the concept of supersymmetry, three supersymmetric waveguides are designed on the basis of supersymmetric optical waveguide pairs. To solve the directional mode coupling problem of supersymmetric waveguides, we insert a slant waveguide in the three supersymmetric waveguides as an adiabatic coupling medium. The coupling process is numerically simulated by beam propagation method (BPM). The effect of the slant waveguide's angle on coupling efficiency is discussed and the optimal coupling condition is obtained. Finally, a three-channel mode division multiplexer based on supersymmetric waveguides structure is designed, which provides a new idea for high-speed short-range optical multiplexing/demultiplexing technology.