Diagnosis
That's the overheat protection tripping. The unit has a safety thermostat inside that shuts everything down if it thinks the internal temperature is too high, usually because the heater was stored on its side and oil got where it shouldn't be.
How to Fix It
Unplug it and let it cool down completely for at least an hour. Then, set it perfectly upright on a level floor—don't have it leaning against a wall or on a rug. Leave it upright and untouched for a few hours so any displaced oil can settle back into the heating elements. After that, plug it back in and try turning it on normally.
If That Didn't Work
If it still triple-beeps, the internal safety thermostat itself might have failed or gotten stuck. There's no user reset for that. You'd need to contact Pelonis support for a repair or replacement, as the unit is correctly protecting itself from a perceived fault.
Heads Up
Always store and transport this heater upright. Tilting or laying it on its side is the fastest way to cause this exact beeping issue when you next use it.