This paper is concerned with matching a fully three-dimensional (3D) viscoelastic numerical simulation with experimental results obtained using a multi-pass rheometer for both an entry-exit slit flow and a cross-slot geometry. The 3D code simulates the time evolution of steady flows using a multi-mode Rolie-Poly constitutive equation. A test polydisperse polystyrene was characterized for both its linear and non-linear viscoelastic response and the rheological parameters were used for the simulation with matching boundary conditions for the flow. Both overall pressure difference and flow birefringence were compared for the entry-exit slit flow and good matching between simulation and experiment was found for the three different depth geometries tested. The 10 mm depth results (depth to width aspect ratio of 6.7:1) also showed that a 2D simulation gave a close match to both 3D simulation and experimental results. The flow birefringence fit between experiment and simulation for the cross-slot case, while reasonable, did not match as well as the slit and the results demonstrate that the cross-slot geometry is very sensitive to the extensional behavior of the melt. In addition, examples of the application of the 3D code are given for a monodisperse polystyrene, where the match to experiment proved as good as that of the test polydisperse polystyrene.