My 3 year old is picking at her nails and cuticles... how do I stop her?

Have him/her wear gloves. Another option is to get nail polish that tastes extremely bad... My friend has the exact same problem and she wears some sort of anti-bite nail polish, it is clear polish. The polish should be a good option because through learning, the trial and error of biting and it tasting gross, he/she will stop.