I think it is too early to determine ADHD, they would not diagnose my son at such a young age. You should talk with your doctor and see what suggestions they have for her. Maybe they can help you get to the root of the problem.
Outbursts like you describe could account for a number of things - anywhere from being a three-year-old boy to all sorts of diagnoses.
Is his sister older or younger?
When did this start? How long has it been going on?
Are there certain events that trigger the hitting?
How is your son's diet?
Any other areas of concern related to behavior?
What has changed in his life recently?
Other questions to consider (but I'll spare asking outright) are related to what's going on at home? What is the childcare situation like? Where did he pick up the hitting? Did he recently see a movie, cartoon, or video game with the hitting?
Sorry I can't be of more help other than just asking questions...
Are you presuming it is ADHD?
As far as questions to ask the Dr. - I wouldn't know what direction to point you in without first knowing more of what is going.
Hope these questions help (a little).
Rory