If you are the owner of the Samsung Galaxy S3 or even the Samsung Galaxy Note 2 you will be happy to hear that you can unlock your handset yourself and it won’t cost you a penny. When you have unlocked the device you are able to travel anywhere in the world with it, or you can switch carriers if you are not locked in on contract.
Unlock Samsung Galaxy S3 (guide)
Those who have bought the Samsung Galaxy S3 in the US with AT&T can only use the handset with AT&T for instance. However if you choose to unlock it then you are able to use the device with any carrier. You could send the phone overseas to a relative and they could use it with whatever carrier they wanted as well. So here is how to unlock the handset.
Put in a different SIM card to the one you use with your carrier.
Switch the device off and on again with the SIM card in place.
Open your dial pad and then type in *#197328640#.
Go to Main Menu >  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options Perso SHA256 OFF > and wait 30 seconds or so.
Then hit Menus and choose Back. You should now see Network Lock. Then choose  NW Lock NV Data INITIALLIZ.
Just to make sure that no mistake is made the sequence to type into the dial pad is Asterisk Pound One Nine Seven Three Two Eight Six Four Zero Pound. You may call the asterisk the star and you could call the pound button the hash key.
Your Samsung Galaxy S3 should be unlocked when you have completed the steps above and it should work with any SIM card from any carrier around the world. As Samsung have used the code for quite a while now, it is thought that it will work on other handsets too, so try it out.