This study investigates the fundamental diffuse scattering mechanisms from three typical building wall surfaces, conducting measurements and model parameterization at 28 GHz and two key FR3 frequencies (8 GHz and 12 GHz). A novel three-dimensional (3D) measurement procedure is proposed to capture comprehensive spatial characteristics, and its effectiveness in improving parameterization accuracy was verified using 28 GHz data. For parameterization, we developed a new method utilizing two dimensions of the high-bandwidth power delay profile-received power and delay spread-thereby fully leveraging the rich information provided by such measurements. Furthermore, we introduce the ER-BK hybrid model, which integrates the Beckmann-Kirchhoff (BK) model's high accuracy and cross-frequency adaptability with the Effective Roughness (ER) model's simplicity, applying it to the building surfaces. Our results show that diffuse scattering at 8 GHz and 12 GHz is highly similar, distinct from that at 28 GHz. A comparison revealed that the BK model provides a better fit for our FR3 measurement data. Crucially, we validated the angular generalization of the parameterized BK model using data from a different incident angle than the one used for fitting. The feasibility of the ER-BK hybrid model was also verified through simulation of the parameterized marble surface.