I just finished with Harman Support and this is what worked for my JBL Charge 3:
1. Press the power button to turn the speaker on.
2. Press both the volume up ➕ and play/pause buttons (>||) at the same time until the speaker turns off (should only take a couple of seconds). This will reset the speaker.
3. Go to Settings on your iPhone and make sure bluetooth is active.
4. Power the speaker back on and press the bluetooth button on the speaker. Once it starts flashing, it should show up on the menu in your iPhone under My Devices.
5. Touch the speaker name and it will begin pairing to the speaker.