Ogre and
Irrlicht are two of the best cross-platform free 3d graphics rendering softwares to ever exist.
CrystalSpace is on down the list a ways but still pretty good. In order to get any of them working on Amiga, we'll need really good OpenGL support with shaders and everything.
I haven't used any of them but I frequent game-developers' websites and Ogre and Irrlicht are the first two names that come to mind for open-source 3d rendering.