A couple of alternatives...
I used to use a NAS to hold the music and build the libraries on each machine individually. One machine was the master and I'd add to iTunes on that one so it would copy the files to the NAS media folder. The other machines I set iTunes to update the library but not add the files to the media library. It's a bit clunky since you have to update music on all 3 machines individually, but it means you don't have to put the same music on every machine which can be handy. A big drawback is that everything is tied to the NAS so it's not portable of course.
The alternative is to use iTunes Match, I switched over at new year and it's a lot easier than the above, and it's portable.