1. The three drop down boxes let you select three primary classes (and their level) for it to display on your profile page under "Characters."

2. If you go to "settings" up at the top, you can select which character it displays under "edit character." I have two left over from the beta 4, so I know it works.

3. From what I can tell your profile pic is stuck as the portrait of your selected character, at least at the current iteration of the forums. It's always possible things will change