Granular flows can exert significant impact forces on obstacles, which must be properly assessed to develop design strategies capable of ensuring structural safety. The impact force depends on several factors, including the velocity and density of the granular mass, the geometry of the obstacle, and the local geomorphology. Various formulas have been proposed in the literature to estimate impact forces. However, most of these formulas assume a two-dimensional interaction, thereby neglecting three-dimensional effects. Such effects may be particularly relevant when plane-strain conditions cannot be assumed for the dynamic interaction, as in the case of granular flows impacting bridge piers. In this study, numerical analyses were conducted to investigate the dynamic interaction between a dry granular flow and a rigid obstacle. Two configurations were considered: a plane-strain configuration and a three-dimensional configuration in which the granular flow is wider than the obstacle, allowing the material to flow around its lateral sides. The commercial code PFC3D, based on the Discrete Element Method (DEM), was employed to account for the dynamic nature of the problem and the associated large displacements. The numerical results obtained for the two configurations are critically compared to evaluate how three-dimensional effects influence the time history, peak magnitude and impulse of the impact force.