Guest,
You should correct the situation by getting to know a dentist and putting in some quality time in his/her office. You mentioned that you have shadowed a bit but don't know the dentist very well -- keep shadowing and ask them for a letter in 2-3 weeks. Put in as much *quality* time as you can (ask questions and try to participate while observing).
Also, can you get a committee letter? Kentucky accepts a committee letter in lieu of the 3 letters, and that would solve your problem right away.
-G
As an interesting sidenote, some schools take the LOR thing very seriously. I have a good friend who received several acceptances last Dec. He chose one of the schools and has since paid his deposits, made plans to rent a condo, etc. etc. Just last week this school, which I won't name, sent him a letter saying they needed another LOR from a science professor. Their requirements call for 2 letters from a science professor, and my friend only submitted one letter from a science professor. They are actually willing to drop him from the incoming class if he doesn't get them another letter, and this is nearly 8 months after they accepted him, and 1 month before school starts!! You would think they would understand he is a qualified candidate (since they DID accept him!). Go figure.