Writing your own 3d engine is pretty trivially easy if you can draw the primitives you want in the plane.
Just transform all the coordinates according to a projection matrix (itself transformed by the camera position) then if you're drawing polygons do hidden surface removal (either by z sorting or a z buffer) this isn't necessary if you just want line drawings (the easiest to do from scratch.)
Make sure you know your Linear Algebra, CG is kind of a bitch without it.