DIAGNOSIS That's the washer's way of saying it can't lock the door. The three-beep error is almost always a failed door lock mechanism or a problem with the latch.
FIX First, unplug the washer for two full minutes. Plug it back in. Manually open and close the door firmly, listening for a distinct click. Try a cycle. If it still beeps, the door lock assembly (often called the door latch) has likely failed. You'll need to replace it. It's behind the door gasket and front panel.
IF THAT DIDN'T WORK There's a small chance the main control board isn't sending the signal to lock. Before you condemn the board, check the wiring harness connection to the door lock for any pinched or damaged wires.
HEADS UP The door won't unlock if there's still water in the tub. If you're stuck mid-cycle, you'll need to manually drain the washer first using the emergency drain hose or drain filter access at the bottom front.