Lan problems?

Im not sure if this is the right place to put it...i have been wondering this for a very long period of time and did some research and no luck with it...i have 3 pcs in my room and they all connected to 1 cable modem...through a router...and when i join a game in starcraft, counter-strike and stuff pc #2 and pc #3 cant join the game pc #1 have created...any one know why its doing this? i tried turning off the firewall on all 3 pcs...and still no luck...