Personally, I'd go with committee and science professor. There's a reason most schools request letters from sci profs. Unless you know your dentist super well and they can speak to a great number of your qualities, a science professor will be the better choice. If a school requires all three, they won't process your application until all are received. Have the additional letter sent directly to the dental schools that require it. Schools understand that you sometimes can't upload all of the required letters to AADSAS, and as far as I know, you are always able to send necessary letters directly to the school. Most schools mention this on their websites, but you can always email any schools to get their recommendation. Again, if you don't meet the requirement for letters, they will just wait until they receive them all. They won't throw it away! If you're really concerned, you can always let the school know your plans, too.