if you feel you child is ADHD
, consult your child psychologist
and occupational therapist, have a opinion about it. normally 3 years old children are active, just compare with other children.
their are several test available on internet, its an informal test, a questionnaire model. it is just to give an idea, not a conclusion.
eg: vanderblit ADHD parents rating scale, or check with DSM IV ADHD criteria