On each computer turn on "Home Sharing", which is in the Advanced menu in iTunes.
Once you do this, your computers will show up on the left pane under "SHARED", which is below the iTunes store.
If you select a computer, a "Settings" button appears on the bottom right, and you can click that to set it up so new files automatically get transferred over. There are also tools there to help you import your library thus far.
Do this on each of your computers, for all your other computers, and you're good to go.