Pack a magazine or two, and then, yes, when he doesn't respond, pull the car over. "It's not safe for me to drive when you are (name distracting/annoying action), so I will wait." And then, get out your magazine and put your attention there, even if you are only pretending.
I have friends who have done this-- the first time, they say, usually takes a while for the kids to believe it's real and quiet down. Also a bummer when "well, we had to stop driving for so long, now we don't have time for (fun destination/playdate) etc. " What's really boring to the kids is just sitting there, on the side of the road, getting NO attention.
Then, once it stops, if it starts again, pull over again.
Frankly, I wish more parents would do this. I have taken plenty of rides with friends where their kids were just screaming the whole time. We do not let that go on. If we have to pull over, you bet you are also going to lose a privilege at home as well. My kid is 6 now, and he's perfectly capable of being quiet when we need him to be, esp in dicey traffic-- but at three, just pull over and ignore the heck out of them.