The files in the Extensions folder are the real libraries. When your application runs, it will dynamically link to libraries found in either your application folder or (more commonly) the Extensions folder. Since the system will use the library in your application's folder first (if it's there), you can experiment with new libraries without disturbing other applications that use QuickDraw 3D.
The QuickDraw 3D extensions installed by the installer are the normal extentions; those found in the Development:Debug folder are the extensions with extra debugging code. They will be slower, but during development of your application, they will be much more helpful in tracking down bugs.
Extensions Stubs QuickDraw 3D QuickDraw3DLib QuickDraw 3D Viewer QuickDraw3DViewerLib QuickDraw3D Accelerator QuickDraw3DAcceleratorLib