You need what's commonly known as a wireless gaming adapter. Once you have the new router up and running, you configure the gaming adapter to connect wirelessly to the router, then connect the controller to the adapter with a short Ethernet cable.
I recommend the Netgear WNCE2001 or the Trendnet TEW-647GA. The Trendnet is available online for about $45; Best Buy and Walmart have the Netgear for around $60. If you are an Apple fan, the Apple Airport Express will do the trick rather nicely too, albeit at about $110.
EDIT: The above only applies if you want to connect the AC3 wirelessly. If you want to keep the AC3 wired, follow Alan's sage advice.