Unlocking the Samsung Galaxy S3 and Samsung Galaxy S4 can be a little troublesome but now with the new leaked unlock code, unlocking the device on your own can’t get any easier.
The leaked code is said to be the unlock code for the Samsung Galaxy S4 but some S3 users have reported that the code worked for them as well so you might want to gove it a try as well.
Start by sliding in a new SIM Card. Restart the device and open up the dialpad. Now, key in the code *#197328640#. Next, choose UMTS >  Debug Screen >  Phone Control >  Network Lock > Options  Perso SHA256 OFF.
You will have to wait for at least 30 seconds before selecting back to enter the Main Menu. From there, you will need to select  NW Lock NV Data INITIALLIZ. Your Samsung Galaxy S3 or Samsung Galaxy S4 should be unlocked now.