How do you put in a dental filling in a 3 year old?

Very carefully: True but just joking. Best way is probably going to a pedodontist or a family dentist that works well with young children. Many times happy gas or some type of mild sedative is wise to calm your child. These early experiences will affect their aprehensions for a life time. Make sure it is as comfortable and unstressful as possible, but if there is a cavity it is necessary to get it taken care.

Anesthetic: Three year olds may have cavities and they need to be treated. The behavior and cooperation of the child will determine how to treat. Some children just need local anesthetic ( novocaine), while others may need some sedation. There are dentist who specialize in treating children (pediatric dentist) but a general dentist may treat these children as well.

Pedodontist: You see the children's dental specialist - a pedodontist. They might use mild sedation for best result and comfort of the child (and the parent !).

Pedodontist consult: Were you advised that your 3 old child has a cavity that needs to be filled? Pedodontists also known as pediatric dentists have extensive training in dealing with this exact situation. There are many techniques to help deal with this situation. Using nitrous oxide\oxygen analgesia or oral sedation are two options but not the only ones.

The same as an adult: The actual "filling" is the same as you would probably have. What is different is having to deal with the psychology of a 3 year old. This is something your pediatric dentist does daily. "fillings" vary in size and surfaces of teeth involved. Some are small and can be done without any local anesthetic. Some are large and can involve multiple tooth surfaces and may require anesthetic.