SDL_RenderGeometry(Raw) are only capable of 2D rendering and the vertexes do not even have a z-coordinate.
Genuine 3D support is available in SDL3 via the GPU types and functions: https://wiki.libsdl.org/SDL3/CategoryGPU
Genuine 3D support is available in SDL3 via the GPU types and functions: https://wiki.libsdl.org/SDL3/CategoryGPU
As long as you can draw 2D triangles there's nothing stopping you from drawing a "3D" scene with a software transform pipeline. Depth sorting gets fun without a z-buffer though.