3 beeps is the HOUR MEMO warning as others mentioned above. Next time you hear it, take a look at the clock...15 seconds after hearing the 3 beeps...the time should change to the top of the hour.
OBC TEST 13 plays all of the Gong's tones...the 3 beeps is the HOUR MEMO warning...is this what you're hearing?
https://www.google.com/url?sa=t&sour...NyKO_6M06TSmkg
You leave no vehicle info (model/model year/build date) listed which might give a clue as to how your X is configured (Nav or MID) because both have an icon illuminated that indicates when the HOUR MEMO is initiated.
You don't have to wait for 15 seconds before the hour when you know what the hour memo icon looks like.
If you have the MID:
{Tone signal is to the right of the hour in pic of MID below }
If you have Nav...again the tone icon for the hour memo is located to the right of the posted hour along the bottom of the nav screen.