We investigated lateral solid mixing in gas-fluidized beds using CFD and DEM. We ran 3D CFD simulations, comparing the results with those formerly obtained in 2D CFD simulations. We observed that the frictional stress model affects the numerical results. This was also observed in 2D simulations, but in 3D simulations the effect is less pronounced. The 3D simulations described the lateral solid mixing process more accurately than the 2D simulations, simulation dimensionality being an important factor. To analyse further the role of frictional stress models, we ran 3D DEM simulations, employing the soft-sphere approach to model the particle–particle contact forces. The simulation results agreed reasonably well with the empirical data, but their accuracy depended on the values used for the collision parameters; also, the 3D CFD simulations matched the empirical data more closely. Altogether, we concluded that the simulation dimensionality plays the dominant role in predicting lateral solid mixing accurately.