You don't have to finish your pre-requisites before you apply for dental school. As long as you complete your pre-requisites before you matriculate (i.e. during your senior year while applying during your junior year). In the summer, I would focus entirely on the DAT as if it was a full time job. A GPA of 3.4-3.6 and DAT score of 19-20 are AVERAGE scores for accepted applicants. You would have a good chance of getting interviews/acceptances.
Lastly, I wouldn't neglect other parts of the dental application: personal statement, work experiences, shadowing, and other extra-curriculars.
Good Luck and slay the DAT in the summer.