Optical metasurfaces hold great potential for near-eye display applications with high optical efficiency, light-weight and compactness. Taking advantage of optical resonances in subwavelength features, metasurfaces can diffract optical beams at RGB primary colors efficiently, forming superimposed virtual images over a real-world scene with merely a single glass substrate in augmented realities (AR) applications. We report for the first time a metasurface with double or triple nano-beams in each period for high angle diffraction with a uniform efficiency at RGB wavelengths. An efficiency as high as 30-40% of the first diffraction order is obtained across field of view, allowing a single piece of AR glass for light in-coupling to deliver image uniformly. The underlying physics is investigated through systematic full-vector numerical simulations. It is found that strong resonances inside the nano-beams with different sizes are the main reason for the deflection of wavefront as well as the Poynting vectors, leading to an efficient coupling of the incident light into the first-order diffraction. The resonances also manipulate the light absorption among the RGB colors for uniform efficiency. This work provides a new understanding of optimal metasurface structure for waveguide couplers using multiple nano-beams.