in this case, why there are 3 available shutter sounds at the menu?
There are two things going on here, and I think we're collectively confused on which you mean.
1. The actual physical shutter makes a light "schlack" sound as it fires.
2. The camera can (I guess) make a digital noise out the little speaker it also uses when you play back video, to add some kind of beep / click / whatever sound, too.
It sounded like you didn't realize #1 was a permanent, physical thing, and thought there was only #2, which you'd be able to silence, leaving NO sound at all. If so, then Bill's right, it's physics. You can silence #2, easily, but when you're using the physical shutter as opposed to the utterly silent Electronic one, you're going to hear the little "schlack." For 100% silent shots, you'd have to use the electronic shutter, and watch out for Rolling Shutter effects.