A low-loss and broadband multimode interference (MMI)-based wavelength multi/demultiplexer in Si3N4/SiO2 technology for erbium-doped lasing and amplifying applications is presented. The structural parameters of a 2 × 1 Si3N4 MMI multi/demultiplexer are optimized to minimize losses. The design and analysis of the MMI multi/demultiplexer are carried out using a hybrid approach, which combines a modified effective index method, the 2D film mode matching method, and the 2D beam propagation method, with lower impact in the computing requirements and simulation time than 3D methods. Simulated total losses of 0.19 and 0.23 dB at 980 and 1550 nm, respectively were obtained for the optimized MMI multi/demultiplexer. The measurements of our fabricated couplers, with 110 nm thick Si3N4 layer, show good agreement with our design. As multiplexers, the average losses of the MMI were measured to be 0.4 ± 0.3 dB for both 976 and 1550 nm wavelengths, and less than 1 dB across the whole C-band. As demultiplexers, the measured average extinction ratio of the fabricated MMI was found to be 21.4 ± 1.2 and 26.3 ± 0.8 dB for pump and signal wavelengths, respectively.