I just had my Gear S3 Frontier stuck in boot loop and I tried the master reset method without success. I was able to fix my watch and this is how I did it:
1. Put the watch on the charging dock
2. While in the dock hold the home button until the reboot screen shows up
3. Press the home button a few times until the menu comes up and press the home button until you highlight power off
4. hold the home button to select the power off option and let the watch power off until you see the charging screen.
If not working, hold two buttons, the screen will show you resetting up still hold til the screen Samsung start as normally.
5. From here try to turn on the watch as normal, if this doesn't work try the master reset method while the watch is on the charging dock.