i would apply a third time only if i have some indication that i can be accepted that third time. if schools tell me that i would have a very strong chance if i do such and such, then i would do such and such and reapply. if you don't get in the second time, it should be obvious why the schools didn't accept you. so you know what you should do. its true that people said if your dream is wanting to become a dentist, then keep reapplying until you get in. but the fact of the matter is, you are also getting older and valuable time is being lost. you might want to reconsider a different career. not all people that want to become dentist will become dentist.
after not getting accepted the first time, improvement on your application should already been made. best thing is to contact the schools, and ask then why you were rejected, and what you can do to make yourself a more competitive applicant. anyhow, good luck, i hope you get one acceptance so you don't have to reapply