Multiplexing technology has rapidly developed due to its ability for parallel information transmission. In particular, 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, the 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 entire WDM system. To address the challenge that conventional wavelength division multiplexers cannot effectively utilize the spectrum due to limited traditional communication band resources, an ultra-compact three-wavelength power-splitting (de)multiplexer (3W-PSDM) is designed using an inverse design algorithm. The device not only performs power splitting and wavelength division multiplexing simultaneously, but also operates at 1310, 1550, and 2000 nm. The simulated insertion losses (ILs) for the corresponding channels are about 3.7, 3.6 and 3.6 dB at 1310, 1550 and 2000 nm, respectively. The simulated crosstalks (CTs) are lower than –23.2, –23.4, and –22.0 dB, respectively. The 3 dB bandwidths are about 56, 54, and 893 nm, respectively. In addition, photonic crystal bandpass filters operating at different wavebands are proposed, which can be connected with the corresponding output waveguides of 3W-PSDM to reduce 3 dB bandwidths. When combined with photonic crystal bandpass filters, 3W-PSDM exhibits that the simulated ILs are about 4.0, 4.8 and 4.1 dB at 1310, 1550, and 2000 nm, respectively. The simulated CTs are lower than –19.6, –15.1, and –26.2 dB, respectively. In addition, 3 dB bandwidths are about 2, 6, and 8 nm, respectively.