once again,, you are half way there
I have a LinkSYS two port router (kind of old) you wouldn't probably find it any more

, I have installed D-Link DI-704P Internet router on many home network system and is seem to be very easy to configure as well as easy on your wallet. Works both with ADSL and Cable modem and has VPN capability to (not a great one but none the less).
A far as firewall goes, if you have enough trust in your friends not to screw around then you wouldn't need to add an over head to your system. BTW, what OS would you install on those three systems ? (Windows XP has it's own built in firewall)
cheers