S.S.
Hey K.,
If your daughter's teeth are the primary reason you are concerned, you may want to talk to a dentist or orthodontist about having a retainer or occlusal guard made for her. As someone who once had braces, and someone who grinds teeth at night, I do know there are devices that can be designed specifically to keep teeth in place.
For the record, there are a number of children who suck their thumb/fingers far longer than three years, so you can take some comfort in not being alone in your fight. I do know a number of these children stop when they enter school and the peer pressure takes over. Is your daughter attending preschool? This might introduce that pressure. Sometimes pressure from someone other than the parent helps.
Best wishes,
S. :)