A:Answer While its not completely impossible, if your plan is to connect to T-Mobile, then you should just get the T-Mobile version (SM-R765T). The hassle and work required to unlock it are enough that buying the AT&T version isn't justified.
For tips on unlocking it, visit XDA fourms
https://forum.xda-developers.com/gear-s3/how-to/guide-gear-s3-frontier-765a-carrier-t3537382