As others have said 3 is WAY too young for that diagnosis and I would be puzzled honestly as to what behaviors cause the parents concern. The problem they might have is a child that has food intolerances and sensitivities to all the chemicals and preservatives in foods, as well as sensitivities to environmental toxins, so their behavior may be very inconsistent. If the body is agitated on the inside, then you will see some reflection of that on the outside. In particular since a 3 year old isn't even close to having the communication skills to indicate when these reactions are triggered, their only outlet is behavior. Gluten and dairy are the top food culprits. They can get tested for those (and soy, yeast and eggs) on their own at enterolab: www.enterolab.com
Or, really, taking that child to the Block Center now may be the best approach:
www.blockcenter.com
Also, if the child has been vaccinated, this behavior can be due to reactions to the toxins in the vaccines - while they've removed mercury, there is still aluminum ( a potent neurotoxin) and things like formaldehyde. And, while they may be in small amounts some people are extra sensitive and have bodies that just don't detox these things.