Start by copying the music files to each computer (either over a network, if your computers are networked, or by burning the music files onto CDs or DVDs – you can use a CD-RW or DVD-RW disc for this to save plastic). If you don’t know where the files are, use “Get Info” on one of them, and at the bottom of the Summary Tab, you’ll see “Where:” and a list of folders. Your music is inside of those folders, so you’ll need to copy (not move or cut) the songs from those folders onto discs or a networked folder.
Once you have added the songs to the iTunes libraries on each computer, you need to authorize the computers with the same iTunes Music Store account information. Just double-click any purchased song in iTunes, and enter your iTunes Music Store user name and password when prompted. You’ll need to have Internet access on each computer. When this is done, you’ll be able to use the music on all three computers, and on all three iPods. Note that you can have up to five computers authorized for your iTunes Music Store account.