Diagnosis
That's the unit's way of saying the water bucket is either full, not seated correctly, or the sensor on it is acting up. Three beeps is its bucket alert.
How to Fix It
First, pull the bucket straight out. Even if it looks empty, dump any water out. Then, look inside the slot where the bucket goes. Make sure the little plastic float valve inside the unit moves up and down freely—sometimes it gets stuck. Wipe the bucket and the contacts inside the unit with a dry cloth, especially the two metal sensor pins. Slide the bucket back in until you hear and feel a solid click.
If That Didn't Work
The issue might be with the bucket's own sensor or the main board. Unplug the unit for a full five minutes. This can reset the control board. After plugging it back in, listen for the startup chime and see if the error clears before you even touch the bucket.
Heads Up
If you're using a drain hose, you still need to have the bucket installed correctly for the unit to run. That click is non-negotiable.