Why unlock your Samsung Galaxy S3 to use with a different SIM card? One possible reason is that you love your phone, but not your cellular provider because of the poor service or an unreliable connection. If you purchased a Samsung GS3 with AT&T, T-Mobile, etc. on a contract, it might be locked for a specific amount of time called the contract period.
After the contract period, you may request your network provider to unlock your phone, but it's not as easy to unlock as you think. In the U.S., unlocking became illegal after January 26th, 2013. However, a new bill is in progress to make it legal again.
To switch network providers, you need to first unlock your device. Many online websites or offline local stores can do it for a fee, but why pay if you can do it yourself?
Here's the step-by-step guide to show you how to quickly unlock your Galaxy S3, any model. If you have a newer Samsung Galaxy S4, check out these step-by-step instructions for SIM unlocking it.
Before start unlocking, make sure you have properly backed up your device. It's risky if you don't properly use the instructions or follow the steps one by one, so it's not my responsibility if something happens.
Your phone must be running on Android 4.1.1 or later to successfully use this method. Check your device Android version by opening the Settings menu, scroll to the bottom, and select "About Device" and check your phone's "Android version" number.
If your phone is not updated to Android 4.1.1 or higher, you can do it by selecting the next menu to "About Device," select "System Updates," and then click on "Check for Updates." Your phone will automatically looked for the update over the network.
Open the dialer of your phone and enter the secret code *#197328640#—it will open the Service Mode Menu.
In the Service Mode Menu, select  UMTS. Tap the menu selection on you phone's screen to select it. If you select the wrong option by mistake, press the MENU button of your device and select back.
In the menu under UMTS, select the  DEBUG SCREEN.
In the Debug Screen, select  PHONE CONTROL.
In the Phone Control Menu Screen, select  NETWORK LOCK.
In the Network Lock Screen, select  PERSO SHA256 OFF and wait for 30 seconds. Then press the Menu button and select back.
After going to back to main menu, select  NW Lock NV Data INITIALLIZ.
Once you select NW Lock NV Data INITIALLIZ, kindly wait for almost one minute and then reboot your phone. Note you didn't receive any confirmation for the process.
Insert the non-accepted SIM Card and power on the Phone. If your phone didn't ask for Unlock Code, it's permanently unlocked to use with any GSM carrier.
You may have to downgrade your firmware version in order to let this method work. alternatively you can call your service provider and ask them for SIM unlock code without paying from your wallet. The only catch is that each service provider has few criteria which must be met before they give the unlock code for your device.
If you do not meet the criteria required by your service provider, a third-party SIM unlock service for your Galaxy S3 is your only other option. Please note sometimes your device may need hard reset to perform unlocking.