Air Force will be more competitive than the other two, at this point the Army will take almost anyone who can get into dental school, but I'd stick with the Air Force if you can.
When I got my scholarship the Army gave automatic acceptance to anyone with a 3.25 (think) GPA and a 19 on the DAT.
Letters of recommendation from current officers help. If you are seriously looking into scholarships, see if the recruiter can set up a time for you to visit a clinic at a local base, then ask the staff if you can shadow somebody for a few days and see how things run day to day. Most places will do anything to help out a dental student, especially if they are a potential recruit and if you spend some time at the clinic I'm sure someone there would write a letter for you.
I did my interview in the recruiters office, but it was cake, simple questions, nothing to worry about.