The explained steps in this Guide will help you to Factory Reset Samsung Galaxy S3 with model numbers:

Samsung GT-I9300. Samsung GT-I9300T. Samsung GT-I9305. Samsung GT-I9305T. Samsung GT-I9308. Samsung Galaxy S3 GT-I9300 Factory Reset Checklist:

1- This process restores the Android platform to its original state and wipe all your data, So backup all the wanted data (Contacts, SMS, Photos, Videos,… Etc).