An unofficial Android version 4.1.1 JellyBean Firmware for Samsung Galaxy S3 GT-I9300 now appears with build number I9300XXDLG4. This has been alleged as unfinished build for upcoming official released soon.
Some advance users, managed to install this ROM and found some part that is still missing, it has no Baseband MODEM and CSC file.
I9300XXDLG4 Android 4.1.1 Jelly Bean Test Build Firmware
Base Firmware: I9300XXDLG4 (4.1.1 JB)
Country: Open Europe
Build Date: 31 July 2012
Change List: 11164
Again, this is not an official firmware released by Samsung. If you want to give it a shot read the whole discussion thread first on the link below before trying to flash this ROM. There are some issues that needs to be truly understand first before attempting to use this ROM .