There isn't really a way to ideally merge them. Methods such as the one linked in an earlier post have you make sure your media are more or less in one or two locations by using the organize/consolidate step for each library, then adding media files from the secondary library to the primary one as new files. If you have a lot of content overlap you will end up with duplicates.
So, use consolidate/organize to make sure you have each iTunes library's content intact in one place on each computer. You don't need or want to consolidate to another drive or computer, just make each one internally complete. Choose a library to export intact. Copy the entire iTunes folder to the other computer. If the third/new computer has no iTunes content simply overwrite the iTunes folder there. If it has content then you'll have to do additional steps but I won't bother with those yet. Start iTunes in the new home and make sure the primary library is working (it should look identical to the original one). Now you can copy the media folder alone from the other computer. Copy it onto the Automatically Add To itunes folder on the new computer. It will add the files as new tracks. If you want to do some hand-weeding of particular duplicate albums you can also be more selective in what you add to the second library.
It is possible to export playlists from the secondary library and then import them into the new library. note that a playlist is simply a text listing of which files are in the list and the files will have to be on the new computer in the identical locations (the point of consolidating).
