By now, many devices have been treated with unofficial Jellybean/CM10 builds, and the Galaxy S3 (both international and variants) are ready to join them. They received preview builds of CM10, all thanks to Team Hacksung. The builds are in beta state, which means they are buggy and not suitable for everyone for daily use. But most of the things are working, including connectivity, camera (has bugs), hardware acceleration, and so on… As always, beware of the risks of flashing test builds, as you might brick your phone. Follow the sources below to go to the official thread of the builds for your respective device.
Source 1 (i9300)