Do I have to replace my battery if the Dp DOCK displays it has less than 3 bars?

No, DermapenWorld suggests that you replace your battery with a fully charged battery if your battery level is 3 bars or below.  

This is the ensure that your device is getting the most power possible from this power source and to ensure you have enough battery level to complete your treatment.  

Please note. You will see a single light on the Dp DOCK panel if you put your device into Bluetooth Mode using the old "Force Sync" (holding the increase speed and increase depth buttons together for 5 seconds). Please ensure you instead use the new 'Bluetooth Mode' (pressing and holding the power button for 1.5 seconds) to put your device into Bluetooth Mode prior to placing the device on the Dp DOCK.