Fix keep shutting down Samsung Galaxy S3
1. Replace the Power button.
Yes some users reported that the issue is related to the damaged power button, it get stuck pressed down, as the result the phone would be in repetitive loop of turn on, turn off and so on.
The solution offered for those experiencing the damaged Power button is simply replacing the button. Here is the Youtube video you can follow to guide you replacing your power button.
2. Remove the back case.
This could also be used to solve the your Galaxy S3 shutting down and rebooting. A user who found the solution said the symptom is he heard the switch scratching on the case when he pressed it. He later found the back cover he used preventing the switch operation, it doesn't allow to pop out smoothly.
He then removed the back case, once the back case was removed, now each time he presses the On or Off button he doesn't hear scratchy sound, and no shutting down issue anymore. He also shaved a little from the case's edge.
3. Take off battery.
Albeit I believe the above solutions will work for you. But still I would like to share the third solution because there is a user that underwent with this one and get his Galaxy S3 fixed from keep disconnecting issue. You may try it. As this is the most simple and not-too technical solution.
- Now take your battery off from your phone. Leave your phone without battery for about ten minutes. Your phone will lose energy from capacitors in this period.
- After ten minutes you can re-insert your battery to your Samsung Galaxy 3. Then turn on your phone.