DIAGNOSIS The door is not fully latched or the door latch switch is faulty. Three beeps is the standard 'door open' alert.
FIX Open and firmly re-close the door. Listen for a definitive click from the latch mechanism. Check for any utensil or dish blocking the door from closing fully.
IF THAT DIDN'T WORK The door latch assembly is likely broken. Unplug the dishwasher. Inspect the plastic latch hook on the door itself for cracks. Inspect the strike plate on the tub frame. If damaged, you need a new door latch kit.
HEADS UP Do not try to bypass the door switch. The dishwasher will not fill with water if it doesn't think the door is closed, which is a safety feature.