Multiplexing technology has been rapidly developed due to the ability of parallel information transmission. Especially, wavelength division multiplexing (WDM) technologies significantly improve the information transmission efficiency by effectively multiplexing and demultiplexing different wavelengths in a single optical waveguide. As the core device in the WDM system, wavelength division multiplexer undertakes the key task of accurately combining and separating optical signals of different wavelengths. Its working bandwidth directly determines the transmission capacity of the whole WDM system. Aiming at the challenge that the wavelength division multiplexer can not effectively use the spectrum due to the shortage of traditional communication band resources, an ultra-compact three-wavelength power-splitting (de)multiplexer (3W-PSDM) is designed by using an inverse design algorithm. The device can not only realize the functions of power splitting and wavelength division multiplexing at the same time, but also operate at 1310 nm, 1550 nm and 2000 nm. The simulated insertion losses (ILs) for the corresponding channels are about 3.7 dB, 3.6 dB and 3.6 dB at 1310 nm, 1550 nm and 2000 nm, respectively. The simulated crosstalks (CTs) are lower than -23.2 dB, -23.4 dB and -22.0 dB, respectively. 3 dB bandwidths are about 56 nm, 54 nm, and 893 nm, respectively. In addition, photonic crystal bandpass filters working at different wavebands are proposed, which can be connected with the corresponding output waveguides of 3W-PSDM to reduce 3 dB bandwidths. By combining the photonic crystal bandpass filters, 3W-PSDM exhibits that the simulated ILs are about 4.0 dB, 4.8 dB and 4.1 dB at 1310 nm, 1550 nm and 2000 nm, respectively. The simulated CTs are lower than -19.6 dB, -15.1 dB, and -26.2 dB, respectively. In addition, 3 dB bandwidths are about 2 nm, 6 nm, and 8 nm, respectively.