Blender for free is a good choice however there is a steep learning curve, if you've already into Autodesk/discreet products I would personally recommend Maya PLE (Personal Learning Edition) which is free but renders out with a watermark.
So what are you trying to achieve anyway although 3DS MAX is a 3D application I wouldn't consider Combustion to be one, it's more of a compositing application IMHO.
Combustion you can get on a Mac however I don't think they've released a native Intel version yet, however a good alternative is Motion or After Effects.