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How to unlock a tmobile galaxy s3 
Get your galaxy s3's imei code. 
Using the on-screen keypad, type in * 06 and the imei code will be displayed on the screen. 
Get your unlock code.
All you need to do is to go to the unlock website and provide the brand, model, the imei code of your galaxy s3, and an email address. They will process your request (which takes only a few days) and will send the unlock code to the email address you provided.