Three RAMPS boards wasted by plugging in a heated bed
It could be a couple of issues:

the heated bed tries to use more power than the ramps board is capable of because the traces on the board aren't big enough to get rid of the heat.

the thermistor is broken / not properly connected or configured, and it keeps on heating the bed forever.

a short somewhere in the wiring.


My advice:
Check the wiring. If there is no short, check your thermistor. Is it reporting the right temperature? If it reports the right temperature, try using a external mosfet module for the heated bed. These are fairly cheap, and easy to connect.