Surface Pro 3 does not support *some* AC wireless routers.
Microsoft Customer support said the only solution was to restore the OS to factory state and disable Windows Updates because one of the updates is creating this problem. Of course this is not a solution since we can't run Windows 8.1 without having Windows Updates. I would gladly keep the Surface on a G router but the problem is that when I travel, it is unable to connect to some routers because of this problem, and if this device has no internet, it is basically an expensive brick.
Too bad because it is much better than the Surface Pro 2 and any other tablet out there, but without a good wireless card and drivers, it is useless.
But even if "The Wireless adapter cannot maintain a stable connection to AC/N Wireless routers" I don't think it's fair to say that the device doesn't support AC routers.