How To Switch Off OnePlus Without Power Button?
When the OnePlus 3 is turned off, press and hold the volume button down for a few seconds.
While still holding the volume button down, connect the OnePlus 3 to a computer using a USB cable.
Wait for your phone to boot to Download mode.
Once it’s done that, press down on the volume rocker to cancel the operation.
After the operation has canceled, the OnePlus 3 will reboot and turn on like normal.
Congratulations. You have successfully turned on the OnePlus 3 without using the power button.