1 make sure all computers are on the same workgroup (win xp - rt click my computer > properties > computer name) (win 7 - search "workgroup" from the start orb.)
2 plug in the hd into one of the pc's. in my computer rt click on it and select properties. sharing tab - press share, and set a name.
3 on the other machines, rt click on my computer - map network drive. browse to the first computer and select the share that you created. it will appear under whichever drive letter you choose.
Thanks for the reply David;
I do not know if it matters but each of my 3 computers are running different OS's. 1 running Windows Vista, 1 running Windows 7, and 1 running Windows XP. Does it matter which computer I pug the HD into?