The performances of a red, green, and blue (RGB) combiner are crucial for the future development of laser beam scanning display systems. In this work, we conduct a comparative study on RGB combiners designed with silicon nitride (SiN), gallium nitride (GaN), and SU8 to reveal their comparative advantages, such as refractive index difference, dispersion characteristic, single-mode condition, transmission efficiency, and light field distribution. Our findings indicate that GaN devices exhibit the largest refractive index difference between the core and cladding, the smallest single-mode cutoff size, and the device length of 2000 μm, and SU8 devices have the smallest refractive index difference between the core and cladding, the largest single-mode cutoff size, and a device length of 3600 μm. In addition, the RGB average transmission efficiency of SiN, GaN, and SU8 devices are 78%, 55%, and 91%, respectively. It is worth mentioning that the optimized ultra-compact GaN multimode interference-based integrated RGB waveguide combiner is suitable for application in micro-miniature integrated projection systems, while its SU8 counterpart holds great value in deformable systems owing to flexibility. These devices will serve as the technical foundation for the development of optical projection systems that prioritize microminiaturization and high transmission efficiency.