Very odd that the thermostat has gone on all three kettles. Atheist, it's the trapped steam that travels down the pipe and triggers the thermostat
"A channel within the kettle, typically inside the handle, carries steam from the top of the boiling interior down to the thermostat (often near the base), where the temperature rises very rapidly from around ambient to near 100°C as soon as boiling starts. This causes the disk to snap and disconnects the power. The thermostat can now be set to trip whenever the water boils, even if this occurs at different temperatures."
https://www.ingenia.org.uk/Ingenia/Articles/24531842-e997-4adb-b023-f19733793727#:~:text=A%20channel%20within%20the%20kettle,snap%20and%20disconnects%20the%20power.
The channel in my kettle is a plastic pipe on the inside of the kettle, it isn't in the handle