DIAGNOSIS That's the dryer's way of telling you the door isn't properly closed or latched. The three-beep error is almost always a door switch issue.
FIX First, check the obvious. Open the door and close it firmly. Listen for a solid click. If it still beeps, you need to check the door latch mechanism. Unplug the dryer. Look at the metal strike plate on the door itself and the plastic latch assembly on the dryer frame. Make sure nothing is broken, bent, or has lint jammed in it. You can manually press the door switch (a small black button or lever inside the latch housing) with your finger while the door is open. If you hear a faint click and the beeping stops when you try to start it, the latch just needs adjusting.
IF THAT DIDN'T WORK The door switch itself might have failed. It's a small electrical component. You'd need to unplug the dryer, access the switch (usually behind the front panel or inside the door opening), test it for continuity with a multimeter, and replace it if it's faulty.
HEADS UP Be careful when working around the door switch wiring. Even unplugged, capacitor discharge can be a risk. If you're not comfortable, it's a straightforward job for an appliance repair tech.