There are physical differences between the Whistle 3 and Whistle GO as well:
1. Size - The two models have slightly different sizes and shapes, so the collar attachments and Twist & Go collars for Whistle 3 are not compatible with Whistle GO. The attachment lug on the bottom of Whistle GO is smaller than that on the Whistle 3
2. How to Charge - Whistle 3 uses a specialized charging base that plugs into a USB outlet. Whistle GO can be charged via a micro-USB charging cable that plugs directly into the device 
3. How to Reset - The Whistle 3 can be reset using the pin-hole on the bottom of the charger, while Whistle GO's reset button is on the end of the charging cable