Two- and three-dimensional computations with a hybrid finite-difference/particle-tracking technique are compared for unstable displacements. In both homogenous and heterogeneous porous media, gravity segregation is much more effective in 3D than in 2D computations. Whether flow is 2D or 3D, the presence of correlated heterogeneities lowers the range of viscous/gravity ratio over which the transition from gravity-to fingering-dominated flow occurs.