In a systematic parametric study 3D MHD flows in expansions of rectangular ducts with different expansion ratios, expansion lengths, and various wall conductivities are analyzed for fusion relevant parameters in order to establish a data base useful for the design of liquid metal blankets for fusion reactors. In currently considered helium-cooled liquid metal blankets the liquid metal velocities are very small so that inertia is often negligible in comparison with the electromagnetic forces. In the core of the flow the major balance of forces establishes between pressure and Lorentz forces while viscous forces are confined to very thin boundary layers along the duct walls. Near the expansion an intense exchange of flow between the upstream and downstream cores with the corresponding side layers is observed. At the expansion a large fraction of the flow is carried by these thin layers along the side walls. This effect becomes more pronounced with decreasing the length of the expansion region. The three-dimensional flow near the expansion gives rise to additional electric currents, which are responsible for higher pressure drop compared with fully developed conditions. This additional pressure drop and the flow partitioning between cores and side layers are investigated in detail depending on the relevant governing parameters.