Normally, you would need a network hub/switch to add a third PC. However, if you have a PC running Windows XP, it is very simple to install a second network adapter (if you have a free PCI slot) and then "bridge" the two Local Area Connections.
Forget about hubs. You need a router. This will give you indepedent access to the internet from all machines, is better future proof, and often includes a firewall.
Cables: If you have your 2 pcs connected directly then (I think) it has to be using crossover cable. For a router, you will need straight cable. THe cables will need to be replaced, or there may be some thingy you put on the ends to uncross them -provided the crossed cables were crossed in a standard way(?)
This thread is now locked and can not be replied to.