As noted above, do a database rebuild before you start. You must be sure that your Aperture 2 installation is perfect. Likewise, be sure you have a complete backup in case you make a hash of things.
Were I thee, I would move the Aperture 2 library to the 4 TB drive first, BEFORE upgrading to Aperture 3. That should give you lots of free space. Then, if need be, you could clean out one of your old 2 TB drives and use that for workspace for converting the other Aperture 2 libraries.
None of this is hard, it just takes some concentration, time, and space. Remember, you will ALWAYS be safer moving the files with Aperture commands, as opposed to Finder commands.
FWIW, I would not "Export" anything; I would only use the "Relocate" and "Consolidate" commands from within Aperture so as to save my internal folder arrangements.
Good luck! This isn't hard, but read the manual carefully and, if necessary, do some tests first to be sure things are working as you expect.
Again, this process only requires patience (copying gigbytes of files takes time) and space (Aperture will make a 100% copy of your library first, before creating the new Aperture 3 library.
A word on nomenclature. Be sure you understand exactly what is meant by "Managed" and "Referenced" libraries. All Aperture installtions consist fo three elements:
++ The Application itself. This belongs on your fastest drive.
++ The Library. This is a package (a kind of foldercontains all of your previews, thumbs, edits, metadata, and all of the database pointers needed to keep the whole mess organized. This belongs on a fast drive.
++ Your masters (now called, I believe, Originals) are either inside the Aperture library package (called Managed) or outside the package, wherever you place them (called Referenced) or any combination you choose. Your masters can be on your slowest drive as they are not read all that often.
Scholars differ on this but, In my opinon, a managed master library is best unless:
++ You want to put the Aperture application on an SSD, or,
++ Your total collection of images is too large to fit on one drive
N.B.2 Should you go to referenced masters, there is a small speed improvement that can be achieved if you use a dedicated HD - that is, a HD that holds nothing but Aperture masters. This is because the masters are written only once and never moved or editied again. (All of the edits actually live in other files.)
Hope this is clear and helpful. (And apologies if you already know all of this.)