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here is the official XXUFMB3 firmware for samsung s3 I9300 included is ODIN siftware for flashing.

Hungry for a taste of official 4.2.1 Jelly Bean on your Samsung Galaxy S3 I9300? Well your in luck, as Samsung has provided official 4.2.1 leaked firmware for the Samsung Galaxy S3 I9300. This latest iteration has a slightly different approach to Touchwiz, and such things as the ripple effect are a bit different and the backgrounds have been changed as well. It's not perfect, as it isn't the final release version but it is quite stable and very usable for day to day use. 

how to is here:
http://www.androidsfinest.org/update-samsung-galaxy-s3-i9300-to-official-4-2-1-jelly-bean-xxufmb3/