They kinda assume if you are applying that you are already interested in dentistry. I think special interest is outside of the other two.
Hobbies is things you do on your own time...activities basically
Extracurriculars are school-related but not classes
and Special interest is probably something like: Civil War buff, conceptual art, etc.
You have an interest in it but it's not like a hobby. You don't go and reenact civil war battles (that would be a hobby i guess) and you don't go out and build 2000 foot winding walls through scottish woods with lamb hair (that would be a hobby instead of just an interest....or maybe more than a hobby.)