"haveing trouble with IP's"

im lanning at my house playin a game called counterstrike. and we have 3 computers all connected to a router and the router of coarse is connect to the cable modem. we cant play cs in the same server at the same time... but we can all play in diffrent servers at the same time.. im wandering if anyone knows how to fix this... the lan is pretty pointless if we cant play in the same server...
ty in advance