In my opinion, the decision of the office to accept your child as a patient should include performing necessary treatment, unless he has an extreme behavioral issue. I am a general dentist who sees many young children and believe he should have at least attempted to treat your child, especially since you told them that you believed that your child had a cavity even before you came into the office. Unfortunately, many general dentists are more afraid of the children, than the children are of them.
I frequently will "brush" a child's teeth, but it is mostly done to demonstrate propper toothbrushing to the parent and establish a relationship with the child. A three year old should not be expected to brush his own teeth in the dental office...or at home. They need parental assistance until they are about 6 years old.
You should try contacting the local dental school, OHSU, or a local community health center where the care should be more affordable.