The Galaxy S3 is expected to receive its Jelly bean upgrade by the end of the month and that official, but leaks always happen and truth be told some are completely unstable and some are near perfect. SamMobile has posted a nice leaked version of Jelly Bean for the Galaxy S3 with the version number 4.1.1. It has a build of July 31st.
The firmware ends with a DLG4 and SamMobile have the new firmware up for download and of course this is at your own risk. The IMPORTANT thing is to note that the firmware will wipe your internal SD Card, at least that is what happened to SamMobile. Anyway, Check out the source link below for more info.