My phone since I got it new shuts itself off randomly up to 20 or 30 times a day. What is up with that happening???
Does it boot fully and run for a while then reboot or does it get halfway through the boot cycle then reboots?
If the former, you cna try booting to Safe Mode and see if it's a 3rd party app causing the issue.
If the later, you may want to try a Hard Reset, see if that helps. Keep in mind, taking it to the Sprint Store will result in them performing a Hard Reset as well. So it doesn't hurt to try as it could be hardware related. But you would want to try running it in stock form, no 3rd party apps installed if you can help it. If it starts booting again, then it's probably an issue with the hardware itself.
This seems pretty weird, to be honest. I have had my S3 since it came out and haven't seen this. The advice by smotrs seems pretty spot on, though I would like to clarify that Corporate Service and Repair stores shouldn't jump to Hard Resets (though it does come down to one many times in the process of troubleshooting the issue). I believe that your best bet is one of these stores as they can run diagnostics on the device and check the hardware. If you would like I can locate the nearest one for you, just let me know!
The fact that this is being reported for two different devices makes it even more strange to me. As well it makes the possibility of an app being our culprit here. Have you tried the steps that smotrs advised? Let me know so that we can move forward if needed!