We present an efficient simulation approach to study the universal mobility behaviour in Si MOS structures with random Si/SiO2 interfaces. Our approach is based on 3D Brownian dynamics in devices with realistic Si/SiO2 interfaces reconstructed from a Gaussian or exponential correlation function. The carrier-interface roughness scattering is treated ab-initio in our simulations and it results in correct velocity and real space distributions. The method is efficient and capable of 3D simulation of the interface roughness limited mobility in small MOSFETs in a statistical manner. After a careful calibration procedure, we reproduce the effective field dependence of interface mobility for μBulk = 1100 cm2/Vs using a random interface with single atomic steps and a correlation length of 6 nm.