well the problem is that they are all done in different programs. not sure why something is modeled using all those different softwares BUT that doesnt help you out so you need to be able to go into all these different programs and find a common filetype that they all will export and read. i use rhino and that os pretty good with opening many different filetypes but when you do many arent usable. i found that step/stp or iges will work well so if you can export everything in one of those then bring them all into rhino you will be able to boolean union all the parts together. if not and you can only export in a mesh like stl then things become more difficult. meshes are a pain to deal with and tools are limited BUT you can still do what you need to in rhino with a bunch of stl's. there is a set of mesh tools in there. bring them all in and do a mesh boolean union on them. its hit or mis though. sometimes it fails.