Guys I also had the same issue. My JBL Flip 3 started misbehaving. It was automatically turning on and turning off. I tried everything from my side but same problem. Then luckily I solved mystery. You have to reset the speaker by pressing (+) And phone call button(which is used to play/pause/receive calls) at the same time. And note YOUR FLIP 3 MUST BE CONNECTED TO POWER ADAPTER AT THE TIME OF RESET. After the device turns off, Turn it back on and before connecting to phone unpair jbl flip 3 from your Bluetooth settings. Hope my trick will work for you guys too.
With Regards
Piyush Dev