Samsung Galaxy is smart but probably you want to secure your Galaxy S3 so that others can not access your Galaxy S3 without your permission. So if you want to setup the unlock password for your Galaxy s3 home screen, you can follow below mentioned steps.
1. First move to home screen of your Galaxy s3.
2. Now tap to the Settings.
3. Now scroll down and select on Security.
4. Now click on Security Lock in that screen.
5. Now click on Password.
6. With the step 5 Select Password Screen opens up. In the password screen enter unlock password you want to have and tap on the Continue.
7. Now in the Confirmation password screen, enter the same password again and tap on OK.
8. If you are looking immediate lock, then be sure that the check box Lock instantly with power key is checked on in the Security screen.
9. If you want to lock your galaxy s3 after a while, then be sure that the check box with swipe lock checked on.
10. Now uncheck the check box lock instantly with power button.
11. Now click on Lock automatically.
12. Now finally give the lock time out limit.
This is it you are done with securing your Galaxy S3. So if your Galaxy S3 is idle for the time limit you set, it will get locked and to unlock it you have to enter the password you setup.