I don't know about the things said earlier posts but there is a system problem with GS3. Gs3 has a file system partition (wherw ROM with all if its blaotware installs) of about 1.5 gb. The Samsung released new kitkat with all of it new bloatware in it goes beyond 1.5 GB that is to be precise 1.7 GB. Samsung has to cut short its all (paid) bloatware in order to keep the KK rom running in GS3 which samsung cannot (afford) to do. That's why they all together decided not to update the device at all. And despite the device already got enough updates that is from 4.0 to 4.3 and that the device was 2 and half year old now also added to th2 fact samsung not taking pain to update this beast.
As far as Hardware is concerned GS3's hardware is more than enough to handle KK ROMs even the latest Android L. Hardware is NOT the issue. It's all about Samsung being economical about it's bloatware income and team management for this update.
Sent from my GT-I9300