What I usually have to do is convert any 3D objects to generic solids. This usually fixes all my export issues into MA 3D. It was suggested earlier to select 'simple 3D' in some of the plugins, and I highly suggest doing this if you want to keep your MA file size down and high FPS in 3D. Remember, all those 3D objects are stored in your show file, and get pushed down your whole network regardless of 3D being connected or not!