You can major in any field; however, I say do yourself a favor and major in biology. By majoring in biology, you will fulfill your dental school prerequisites and your major's courses at once. Anyhow, you are going to take science classes such as biology, chemistry and physics as dental school prerequisites. Why not start taking them earlier and get used to them? There are more intense science courses to come in dental school, and majoring in biology or anything close to it like chemistry will make your much more prepared for them and also your for your DAT. Speaking generally, people majoring in science [such as bio, chem, etc] do better in their DAT [There are exceptions though]. This way, you will also save yourself a couple of years compared to majoring in arts, business or similar majors. Do not forget shadowing a dentist for a good amount of time, and checking out if you like the job.
If you study hard through the first three years of college and earn good grades in your science classes, you can get yourself prepared for the DAT in about 3 months [studying at least 6 hours a day].
Yes, you can take the DAT three times, but you have to wait 3 months each time you take the DAT in order to take it again. Taking it for the 4th time needs obtaining a permission from ADA.
