Be prepared for a lot of "he's a normal boy" feedback and anti-ADHD talk. I have found, though, that people who suspect ADHD tend to be right on in their hunches.
I have a son who was kicked out of preschool at three for signs of what we now know is ADHD. He couldn't sit still at all, couldn't follow instruction, would kick, hit and spit at kids and teachers (and me), couldn't focus, would run across the furniture non-stop at home, didn't respond to praise or punishment, didn't respond to traditional parenting techniques or consistency. It was off the charts BAD. Some people see the positives in ADHD, but for us it was a nightmare for a long time.
We started behavioral therapy at three, but this didn't help. Our son simply couldn't listen or sit still for any technique to work. That's when he was kicked out of school. When he was four and after some agonizing, we tried Ritalin. Wow! Amazing. Beyond positive, it transformed his life. He was pleasant to be around, focused and yet still was himself (no personality change in any remote way). He was able to join another preschool and has excelled in school ever since.
I would check out the website for ADDitude magazine. They did a story about ADHD in preschoolers a few years ago that might be archived there. Also look at the website for CHADD, which also is a good source of information.
If you want answers, get your son in with a specialist. A pediatrician can't diagnose but specialists like developmental pediatricians, psychiatrists and neuropsychologists can (you aren't likely to get a diagnosis at three, but you can still get help from these professionals. Kaiser said our son had "strong indicators" of ADHD at three and did help us.). You may already be familiar with this type of professional, so tap into your existing resources.
Good luck!