A user has sent me a collada export from another 3D product and it appears to be a collection of .dae files which make up a single model. Is there a way to import these as a group to create a single 3D PDF?
It depends what version of Acrobat you're using.
The result of any of the above will depend on the registration of the parts being correct (so a file containing a screw lines up with the other file containing the part with a hole). If they don't register then you'll need to position them all by hand in 3D Toolkit or 3D Reviewer.