M.J.
As the parent of a child with ADHD, I urge you not to dismiss your concerns. I got the "typical boy" comment all the time and it just didn't seem right to me ... it was clear to me our son wasn't typical boy.
If he has a hard time focusing and sitting still in a variety of environments, that can be a big tip-off. I wouldn't compare his behavior to that of girls, but watch other boys. At three, I was being overloaded with complaints from preschool, life was challenging with him at home and he was the kid running around in the gym and running through circle time at his gym class (and laughing as I chased him) while every other child sat.
You can get your son assessed at this age, so don't let age be a factor in downplaying your concerns. If your gut says something's not right, push for the evaluation.