Okay, you've got a valuable clue with the time frame between restarts. If you figure it takes about 15 seconds to boot to the operating system, then that leaves exactly three minutes before it reboots - which just so happens to be precisely the amount of time that elapses between the SOC scanning its sensors.
What I'm pretty sure you'll find is that the processor is finding an unrecoverable error in one of its sensors. The recovery strategy employed by iOS is to restart the processor in hopes that will reset the affected system and allow it to run. That restart mechanism is called a kernel panic, and is logged in the phone so you can find out exactly why it's restarting.
First thing to do is to go to the Analytics page and locate any current panic logs. When you find them, pull up the latest one and either screenshot and/or copy/paste the first page and add it to your question. That page will have the vital information you need to determine why the system is rebooting. Note that it could be related to the power button, but that's far from the only cause.
Adding images to an existing question - iFixit Repair Guide
Go to @flannelist's excellent Wiki article on Kernel Panics and read up on them. Follow the instructions to retrieve the panic log and once we can see it we'll have a much better idea of what you'll need to do to fix your phone.
iPhone Kernel Panics - iFixit
Keep in mind that you've got exactly three minutes to locate the panic log and take your screenshot/text copy, which is very doable.
Ah, okay so it's not booting; that does change things.
Disconnecting the power button was a good first step and that reminds me of a common problem with the iPhone 7 where it would go into a similar boot loop upon failure of the front camera assembly. So basically I'd suggest disconnecting that and seeing if it affects the boot. If that one doesn't do it, try unplugging everything possible, one at a time, to see if you can isolate the source of the problem.
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