Is my 3yr old is to little to get laughing gas at the dentist?

No. Nitrous oxide (laughing gas) is safe to use on a 3 yr old. Your dentist is trained to use this for children as well as adults. But your child should not have decay at that age. It is completely preventable at home. The bigger picture question is, ... How can we can avoid future cavities and need for additional treatment. Talk to your dentist about this.
Not necessarily. There are many young children that react well to nitrous oxide. It depends on the individual child.
Is safe. Nitrous oxide ( laughing gas) is safe to use on children. It works well with good cooperation. At age three it is sometimes difficult to have the child wear the mask. Use positive talk about the ( magic astronaut mask").