S.M.
I sucked my thumb until I was 12, and I am the only one in my family who was a thumb-sucker, and the only one who didn't need braces. What got me to quit was that my mother told me that if I quit sucking my thumb, she would quit smoking. It worked like a charm. Seeing as your daughter is only 3 years old, try putting the pepper-flavored nailpolish on her thumbs. A friend of mine did this for her 4 year old and he stopped within a few weeks. Good Luck!