No, if you decline your first preference you don't get another offer.you can only recieve one offer at any one time, so no you wont get three letters/offers. you will only recieve an offer for your highest ranked prefrence for which you qualified.
if you qualified for your 1st prefrence, you will get that offer.
if you then decline that offer, you will instantly and automatically get an offer from your 2nd highest ranked prefrence for which you qualified
and so on and so on..