A little advice on letters....
If the application calls for 3 letters, you need 3 letters.
Your letters need to be written by people that know you. It gets pretty obvious when a letter is written by someone that has no clue who you are. However, its also obvious when you have a family member write a letter. Try to get someone else to write the letter.
Letters don't have to come from a dentist or from a military member, but it can't hurt. \
Give the person writing the letter some info about you, so the letter doesn't look so generic.
If you have worked with someone in undergrad or dental school on a research project, and they would write you a good letter, ask them to write one.
A rule of thumb I always tell people on asking for a letter of recommendation. When asking for a letter, don't just say, "Will you write me a letter of recommendation?" It may not be a good one. Always ask, "If I asked you to write me a letter of recommendaiton, would you be able to write me a good one?" If the answer is "NO", go somewhere else.