The main board on the Pre 3 has markings which indicate there are four different versions: Verizon, AT&T, EU and TAB (don't know what that one is), so I assume the board circuitry is different, since the modem firmware is on the flash memory daughterboard.
I have a new AT&T Pre3 sitting here, and am on Sprint (currently running a FrankenPre2).
The Verizon Pre3 has a SIM slot which means it's got both CDMA and GSM capabilities. What I want to know is if the AT&T Pre3 also has CDMA capabilities. In other words, did they just build one phone type (Like iPhone 4S)? If so, that means all we have to do is figure out how to change the firmware/tokens/doctor, then flash a Sprint MEID into it.
This is doable on the old Qualcomm MSM6801 used in the Pre/+/2, but I have no idea about it on the new chipset in the Pre3.