DIAGNOSIS The control unit isn't receiving a proper 'door closed' signal from all the interlock switches. The triple-beep is a standard error sequence.
FIX Press firmly on the door, especially near the latch area, and try Start. If it works, the door/latch is misaligned. Check for food debris or a small object obstructing the door seal. Clean the door seal and frame.
IF THAT DIDN'T WORK One of the three door safety interlock switches has failed. They are mechanical switches that activate in sequence when the door closes. A failed switch must be identified with a multimeter and replaced, which requires disassembly.
HEADS UP Never bypass or tamper with the door interlock switches. They are critical safety devices that prevent the microwave from generating radiation when the door is open.