Standard USSD GSM code for IMEI:
*#06# Shows IMEI (International Mobile Equipment Identity) number
Others that are specific to S3:
*#0*# LCD Test Menu
Dial the secret code *#0*#. Now you get a screen with title LCD TEST. You have lots of option to test various hardware parts of your phone such as speaker, sensor, lcd, etc
*#8999*523# LCD Brightness
*#1234# To check Software and Hardware information, PDA, CSC, MODEM (Firmware Version)
*#2222# H/W Version
*#*#4636#*#* user statistics and Phone Info