Been covered a few times but i've never added my six eggs...
3 beeps seems to happen for 2 reasons, both down to the pressure sensors at the top of the A-pillar;
1. Intentionally. Turn off the sensors (eg when leaving the dog in the car) by opening drivers door, turn ignition to 2nd position-off-2nd-off, close and lock. 3 beeps tells you this has worked.
2. Unintentionally. Sensors go off while locking the car, ie the air's still moving in the car from slamming the door and locking immediately. 3 beeps lets you know. Used to happen to me all the time til i pointed the sensors more to the middle of the cabin and now walk a couple of paces before locking.
I THINK either way the car will be locked with the interior pressure sensors deactivated.