If you already have the router set up on your network, and it is just a matter of hooking up 2 other PC's, it SHOULD be no problem at all.
Is the router (or will it be, if you don't have it yet) be set to do DHCP, or are you going to do static IP addresses? If this is the case, check to see what the range of the IP address pool is in the router. This 'IP pool' is a group of IP Addresses that the router will assign out to PC's that send it a DHCP request. If you only have it set for 2 IP addresses in the pool, and you have 3 PC's, then obviously, 1 of them will not be able to connect.
Generally, router default address is 192.168.1.1 for the LAN. The IP pool generally starts with the address 192.168.1.100. Simply make sure the pool is large enough for the number of PC's you will have connected. STEP 2: make sure that each PC is set to get its IP Address automatically from the router.
