DIAGNOSIS The door switch assembly is faulty or misaligned. The three-beep alert means the microwave thinks the door wasn't closed properly during the cycle.
FIX Inspect the door latch area. Look for food debris or a broken plastic latch hook. Clean it out. Open and close the door firmly several times. If the beeping persists, the door switch (part number WB27X11333) needs replacing.
IF THAT DIDN'T WORK The main control board might be misreading the signal from a good door switch. This is less common but possible.
HEADS UP The door switch is a safety component. A faulty one can allow the microwave to run with the door open. Do not bypass it. Replace it immediately.