DIAGNOSIS The three-beep error usually means the door isn't being sensed as fully closed, even if it looks shut. The door latch or sensor isn't engaging.
FIX First, check the obvious. 1) Open and firmly re-close the door. Listen for a distinct click. 2) Inspect the door latch itself (the hook on the door) and the strike plate (the part it catches on the dryer body) for any damage, lint buildup, or obstructions. Clean both with a dry cloth. 3) Look at the door seal (gasket) around the opening. If it's bunched up, folded, or has a tear, it can block the door from seating fully.
IF THAT DIDN'T WORK The door switch, a small electrical component behind the front panel that gets pressed when the latch engages, might have failed. You can test it with a multimeter for continuity, but replacing it requires unplugging the dryer and removing the top and front panels.
HEADS UP Never try to bypass or tape down the door switch to make it run. That safety feature is there to stop the drum immediately when the door opens. A faulty switch is a straightforward and relatively inexpensive fix.