Easiest way would be to pick one of the machines (which you have done), and drag the folder that contains all your music (or subfolders of all y our music) into the iTunes Library window. Once you do that, iTunes will copy the new music (or old music, in your case) into the new library. I don't think that it overwrites files, because when I've purchased a song from iTunes, and then purchased the album later, it has allowed two versions of the same file to exist. It just names the files differently on the backend.
Once you have your libraries all merged, run the command in iTunes to check for duplicates. Then, at your own pace, you can delete any copies of the same songs that you have.
Before you do anything, back up the library that you are copying everything else into, just in case something goes wrong during the import process. It sounds like you have a lot of files, so make backups of all three (two of which you have already backed up) libraries first.