The milk teeth also need to be treated the same way as in the case of adults. As the age of your daughter is just 3 yrs, dental treatment is a must in case there are any cavities.
Her front teeth will fall and permanent teeth will erupt around 7-8 yrs and the canines and molars will fall and permanent will erupt around 12-13 yrs.
So there is a lot of time still for the permanent teeth to erupt and hence treatment is necessary for the milk tooth.
Along with treatment certain preventive procedures like fluoride application, etc also help in reducing the tendency of decay.