And yet, it got better for awhile. I don't believe you posted about his behavior in over a month - except for the questions on his meds for asthma. By the way, had he taken any on Sunday? - they can really make a kid hyper.
Anyway, if things did get better during the last month. Remember that and what you did. He is 3, changes will not happen overnight! If you haven't done so, look at some of the posts by other parents about their 3,4,5, and 6 year olds. You are certainly not the only one having these problems! Doesn't make it any easier for you, but I just want you to know that at least some of the things he is doing (particularly the stuff in your last lines), are not uncommon behaviors. As an example take a look at this post and in particular Julies comments - she has some great suggestions.
http://www.medhelp.org/posts/Child-Behavior/EXHAUSTED-MOM/show/1288249#post_5899756
As to what happened on Sun. The problem is that you weren't there. You really don't know what started it. I don't know how many times he has been in the jeep, and how familiar he was with it. Its possible that being only 3 he was just heading for an open spot to drive and got really ticked off when he got stopped. In my opinion, you really can only deal with what happens when you are around. I can definitely tell you that punishing him after the event (unless you live really close) was kind of useless. At that age, it's got to be immediate (and of course, consistent). Anyway, I am sure the whole matter was embarrassing and frustrating for you!
I should add that telling him not to do something and expecting an intelligent response is not going to happen until he is about 9 (and by then, he will tell you what you want to hear if he has any intelligence). If he hits a child - its an immediate timeout. Its quite ok to tell him why he is in the timeout, just don't expect a response.
Think about your successes. Remember that it takes about 3 weeks of constant behavioral modification to see a lasting change. This will not happen overnight. Good luck