Router will not reconize 3rd computer

It is possible that the router has a wireless security enabled into it. The laptop needs the security key for it to connect to the wireless network. You can check the security enabled in your router from your router's User Interface page.

To access your router's page; launch IE from a computer that is working through the router. Then on the address field, type 192.168.1.1. It will open a window asking for a username and password. The default password to get into the router's page is admin.

Then, once you're in the router's User Interface, the main page that will open is the SETUP, beside it is WIRELESS tab. Click on it, then verify the wireless network name or SSID. Whatever name configured on it, is the name the laptop should connect to.

Then, there is a subtab under the main tab WIRELESS. The sub-tab is called, Wireless Security. Click on Wireless Security tab and then look for the netwok key enabled. Take note of the key which your laptop will be neeeding.

Hope this helps!