[I'm assuming this will work for ANY carrier, but we all know what assuming does...]
Take it to dealer and have the MEID activated.
Flash the PRL of carrier you want to switch to, restart phone.
You'll also need to change the SPC on the Phone to 000000
Here's the trick, you need another phone with the dial pad open and ready.
hold the other phones speaker to the mic of the S3
Dial *228 on the S3 you want activated.
When it asks you to press 1, press one on the other phone,
The S3 will hear the dial, and proceed to program it for you.
[Back in the day if you recorded the tones that were made from a payphone after paying, you could play it back,, it would hear the dial, and think you paid, when you didn't]
Now Flash a correct PRL [I have provided a copy I used for Page Plus]
No Roaming Signal
CDMA - EvDo Rev.A
Blam, it works.