Reliable Vehicle-to-Everything (V2X) evaluation requires propagation models that represent terrain and multilevel infrastructure. Most integrated vehicular simulators still rely on planar models, but the magnitude of the resulting bias is unclear. This study quantitatively compares flattened two-dimensional (2D) and terrain-aware three-dimensional (3D) variants of three scenarios using identical Sionna RT settings. The pipeline combines OpenStreetMap and digital elevation data, Blender/Blosm scene construction, and SUMO mobility traces. The scenarios represent a 4 km mountain pass, an overpass bridge, and an underground tunnel. Path gain is computed at 5.9 GHz; received signal strength and link outage are then derived using a 23 dBm transmit power and a −94 dBm receiver sensitivity. The outage rate increases from 21% to 30% for the mountain pass, from 20% to 33% for the bridge, and from 0% to 80% for the tunnel with 3D geometry. Mean signal-loss discrepancies reach approximately 9 dB for the mountain pass and 10 dB for the bridge. GPU acceleration reduces per-frame processing time by approximately one order of magnitude in a controlled bridge benchmark. The results show that 3D-aware propagation modeling is necessary for credible assessment of V2X applications in environments with elevation changes.