For many important aerospace and marine applications boundary layer separation is three-dimensional (3-D) and already the mean-flow topology is considerably more complicated than for two-dimensional separated flows. In addition, because of the high Reynolds numbers involved, separation is always associated with considerable unsteadiness. This unsteadiness is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean flow. The present investigation aims to gain insight into the physical mechanisms governing 3-D separation. Towards this end direct numerical simulations are employed to investigate 3-D separation bubbles on a flat plate generated by the proximity of a 3-D displacement body. The main objective of the present work is to investigate how the flow physics of the separation bubble change with an increasing aspect ratio (AR) of the displacement body and how the two-dimensional limit is approached. The aspect ratio is defined as the ratio of displacement body width to length. Displacement bodies with aspect ratios up to 10 are considered here, while an axisymmetric displacement body (AR=0.5) served as the baseline case. As the aspect ratio is increased, the separated region gets stretched in the spanwise direction. For very large aspect ratios a changeover from closed to open separation was observed.