You'll need a wireless gaming adapter - basically, those devices that were used to put Playstation 2s on a wireless network, since the game consoles didn't have wireless support back then. Just do a search on Ebay for "wireless gaming adapter" and it should bring up a bunch.
If you want a specific brand/model, I'm using a D-Link DWL-G820 with my AC3. Bought it off Ebay for $27. Works really well.
Something else to think about as I ran into this when selecting an adapter: do you have encryption/security on your wireless network? For some reason, a majority of these adapters only supported WEP and didn't support WPA (which is what I run). Even the D-Link model that I got had two different versions - one didn't support WPA and the other did. So I had to make sure and get the version that did.
That should be all you need (plus a network cable to go from the adapter to the AC3).
