DIAGNOSIS The microwave is detecting the door isn't fully closed or the latch switch is failing. Three beeps is its standard 'door open' alert.
FIX First, check the door is firmly shut—listen for a solid click. If it seems fine, inspect the door seal and latch area for any food debris blocking it. Wipe the inner door frame and latch clean with a damp cloth.
IF THAT DIDN'T WORK The door latch microswitch might be faulty. Gently press the door latch mechanism with your finger while trying to start it. If it works with pressure, the switch is worn and needs professional replacement.
HEADS UP Don't try to operate it if you suspect a faulty switch—the microwave won't run with the door 'ajar' for safety, but a failing switch can be intermittent and frustrating.