My JBL Charge 3 turns off by itself.

My charge 3 recently had the same issue after some weeks of inactivity, the speakers would just shut down almost instantly without reason (full battery level indicator and it charged correctly so it didn't seem likely to be at fault here). Then I realised the battery sensor problem that appeared a few days ago on my Android phone (battery level status showing negative digits) could be causing the connected device to turn off, and so it was. Try pairing it to a different device and see if this solves, in that case I would also hard reset the speakers (press and hold volUP+play/pause until it turns off) to avoid automatic connection to the faulty device, notice that this will clear the memory of all devices paired in the past.