First, you need to question yourself why your child has so many cavities at
age of 3. Have you been taking good care of his/her oral hygiene? For a 3
years old, most "back" teeth won't lose till 11 years old. So of course, the
cavities needed to be removed asap. It will be hard for the child to
cooperate, so laughing gas or even general anesthesia is required.
If you think $500 is too much, next time about getting tooth brush and floss
for your child.