The 3-channel directional coupler is a device composed of 3 waveguide channels. In this study, the 3-channel directional coupler was applied as a long-wavelength light filter. The performance test of the directional coupler as a light filter was carried out in a simulation using the Fourier Transform-based Beam Propagation Method. The simulation results of the 3-channel light filter directional coupler test showed that the long-wavelength light filter can be performed well on the channel switching process by light waves. By using the right 3 channel directional coupler design parameters, during the channel switching process, light waves with long wavelengths can be separated from light waves with higher frequencies. In this research, the design of a 3-channel directional coupler is composed of glass as a film layer, acrylic as a substrate layer, and air as a cover layer.