The following is a guide to unlock the T-Mobile Samsung Galaxy S III with Z3X box. I have verified the steps below it worked for me. It's not easy to get it right because of lack of information available elsewhere.
- Connect Z3X box to PC via USB.
- Enable USB debug on the phone.
- Dial *#7284# on the phone, choose Quadcomm USB settings -> DM + Modem + ADB
- Connect T-Mobile Samsung Galaxy S 3 T999 to PC via a different USB port.
- Select model T999 in samsung tool 13.1.
- Select the USB port and then click unlock. The phone will reboot itself after unlock is done.
- Congratulations, now you have just unlocked your T-Mobile Samsung Galaxy S3!
To verify whether it's unlocked or not, just pop in a sim card from another carrier (for example, AT&T), and reboot the phone. If the phone is not unlocked, it will prompt you for unlock code upon startup.
Does it work for you? Please leave a comment if this helped you unlock your phone.