Three UK has confirmed that the Android 4.1.2 Jelly Bean update for the Samsung Galaxy S3 is now available for download via the PC companion software Kies. If you have no idea on how to update your device through Kies, you can refer to the tutorial on Samsung’s official website. However if you do not like to manually update your device this way, you can also go to Settings > About Phone > Software Updates to scan for the OTA update rolled out by Three UK.
So have you updated your Galaxy S3 to Jelly Bean?