Oh I see your problem, the schools that I applied to didn't require a LOR from a dentist so I submitted 2 from science profs and 1 from my supervisor. I think the limit for number of LOR's on AADSAS is 3 or 4 I forgot. So if you want to be sure just submit 2 from science profs and 1 from a dentist because some schools need one from a dentist and some need 2 LOR's specifically from science profs. I guess it doesn't hurt if the schools that only need 2 received 3 letters from AADSAS but will definitely hurt if they need 3 and only received 2. Some schools will follow up with you to submit secondary application or additional documents if they need more.
Hopefully someone will answer your question better.