To hard reset your Whistle 3, you'll need to do the following:

  1. Place the device on its charging cable (orientation doesn't matter) with the cable plugged into a computer or an AC Adapter.
  2. Take a paper clip (or some other thin tool) and on the charger insert the end of the paper clip into the RESET button located underneath the center of the charger. You should feel a slight click when the button is depressed. 
  3. Hold down for about ten seconds (at least 5 sec)

During this time the LED on the Whistle 3 will turn off, and once you have released the button, the light will turn on either green or amber within a few seconds (depending on how full the battery is).

Here's a video of what to expect: