We have found that Joyrides work well for the dcs, both of whom get a bit travel-sick. This makes picture books a possibility.
At three, they both liked stickers, which are not too messy (well, my two could generate mess in an empty aircraft hangar but sticker debris can be cleared up fast).
We have a lucky dip bag, which they aren't allowed to see in advance, and which usually contains stickers, little action figures, Duplo figures and a few Duplo bits like Dora's seesaw, a swing, a trike, boxes of raisins, a couple of packs Organix cheese puffs, bottles of water - some new, but most of it already in toy boxes at home. Also mini-books of stickers, crayons and some Crayola or Galt small-format pads.
That goes on the back seat between them and they forage.
Depending on whether he is just 3 or almost 4, you might find I Spy and Spot the Red Car type games a possibility...
Good luck - I have to admit car journeys are a lot easier now that mine are 4 and 8.