to link three or more pc's together the best way would be to use a hub, but for this you would need straight (sometimes called Patch) network cables which means replacing what you already have working.
i dont know if it is possible to network another (third) pc by just adding another network card to the main pc (pc1) and using a crossover cable as you have done to pc2. maybe someone with more knowledge will jump in and elaborate (think its spelt right lol).
if you are looking for a setup which would allow pc3 to access the internet without other pc's being on then you would need a router. kinda gets confusing dont it!!
good luck, just wait a while for some more advice