Your focus on touching himself in private is the correct guidance. It's best not to characterize the beavior as 'nasty', because the behavior per se is fine. It is unusual for a child so young to engage in this behavior as much as your son does. Is he otherwise developing OK? Are there any other concerns?
In light of his ADHD, he may be more impulsive than the average child his age (though it is normal for children his age to ne impulsive). Thus, he may require a bit more limit-setting than the average three-year-old.
He's a very smart child; completely potty trained at 1 1/2, holding great coversation with an adult at 2, an amazing memory. He's developing accordingly from wat his pediatrician tells me. He is ADHD though, and already on adderall for it. (Young I know, but he needed it. He was about to be kicked out of daycare #2.) But other than that he's normal. (Alittle small for his age though.)