It was implied that the S3 will get KitKat. Without finding the exact reference, it was stated that region locking was going to be included with the KitKat release for the S3. Now, since KitKat itself was released by Google in 2013, we are within the two year window for releases, so that also is a positive. Keep in mind that the S4 has not gotten KitKat yet, and that one is clearly expected to get it, so the lack of any official statements isn't a bad sign at this point.
Now, the S3 on AT&T SHOULD be released on or slightly after November 13th. Remember, 4.3 is needed for the Galaxy Gear, so Samsung put in a fair amount of resources to get the carriers to release 4.3 SOON, as in, before the holiday shopping season.