Have a Pi3 Model B, was playing around with GPIO and trying to integrate a PIRO sensor (IR). Must have done something wrong with the circuit as now the Pi immediately reaches 100+ degree Celsius within few seconds after booting with a thermometer icon on top left of the screen. How to fix it?
The Pi 3 should throttle its CPU down when the system temperature reaches around 80-85 degrees Celsius. If your Pi doesn't do this then it's broken. As a last ditch effort you might consider trying a small heatsink and/or running a fan over the board, but it fundamentally shouldn't be getting that hot.
I think it's probably time to return or junk the Pi. Trying to fix it is going to be more expensive and time consuming than makes sense in the context of a £35 device.
The system should not get that hot. However (as the last step before throwing the device into recycle bin) I would try to boot from clean unmodified system to be sure there is no software-related problem there.