The Samsung Galaxy S3 comes with Android 4 (aka Ice Cream Sandwich) preinstalled. You either mixed it up with "Jelly Bean" (which is Android 4.1), or you want to install a stock Ice Cream Sandwich.
For Jelly Bean you should wait a few weeks. Samsung will most probably provide an update for its latest power horse.
For stock ICS, you should refer to XDA Developers forum. There you'll find several flavors for the SGS3.