A flashing red LED light on the pi-top [3] Hub would indicate the Cooling Bridge does not have a secure connection with the Raspberry Pi pins and/or the pi-top Hub.
Ensure the pi-top [3] is completely powered off before you perform the following checks:
- Check the pi-top [3] Power Supply Unit is fully inserted into the back of pi-top [3] unit.
- Fully insert the Hub audio jack and USB into the Raspberry Pi.
- Check the Cooling Bridge has the correct Thermal Pad affixed to the Heat sink
- Ensure the Cooling Bridge has a secure connection and aligned correctly onto the Raspberry Pi pins and Hub.
- Insert the microSD card the correct way up into the Raspberry Pi.
- Finally, check the Cooling Bridge screw has not been over tightened.