Basically it's up to you. The safest is to get 3 science letters, as a few schools require specifically 3 science letters. However if you are not applying to a school that requires this, you can get one from a dentist or a non-science professor, either is typically fine. Another option is to get 4 letters (the maximum allowed on AADSAS), where you could do 3 science and a dentist or non-science.
Examples of what school's require:
Columbia: Three CONFIDENTIAL letters of recommendation from science professors or one composite letter from a pre-health committee. Additional letters of recommendation from employers or non-science professors will be accepted, but will not satisfy the requirement. Official letters of recommendation are only sent directly to the AADSAS application service.
UOP: Three letters of evaluation. If your undergraduate institution has a pre-health advisory committee, a committee evaluation is recommended. Otherwise you must submit two letters of evaluation from science instructors and an additional letter from a health care professional or employer.
OHSU: OHSU requires two letters of evaluation from basic science faculty and one from a dental professional, or one pre-professional committee letter. If your pre-professional committee letter does not include an evaluation from a dentist, we ask that you send a supplemental letter of evaluation from a dentist under separate cover.
And on and on. Hope this helps.