Make sure you're plugging the third PC into a switch port on the router, and not the WAN or DSL Port.
Also, do an IP Release/Renew on the third PC after connecting to the Router and getting a link light. Please post if you need help with releasing your IP address.
I have a linksys router at home with two PCs connected and sharing Internet access. I made a network cable to connect a third PC but when I connect to router and PC I see the connection status is connected at 100.0 Mbps but no Internet.
I swapped the cable and everything worked fine, I was able to access Internet. The funny thing is, if I use the same cable I made at work on our network I can connect to Internet with no problems. Any ideas why that is? Thanks!

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