I really don't think it's possible.
In all my years I only heard of one person who had more than 2 specialty training completed. 2 specialty are rare but they do happen, Perio and Endo, Peado and Anaesthesia. The person with 3 was not engaging in the practice at all, but was in academia full time.
The problem would be admission into the 3rd specialty. Which admission commitee member would allow somebody who doesn't know what they want. Do you want to do Pros and Ortho now Pedo. "Come back and apply when you know what you want" would be the response right before "don't call us, we'll call you".
Looking at the fact that the OP is a premed, we know that there is no choice made yet as to going med or dent route.