I like Joe's answer, but will contribute a few more, also based on extended empirical studies.
Stuff to Bring
Lots of Paper and Crayons
Buy a few coloring books, connect-the-dots templates, mazes, etc and a lot of spare paper and crayons. If that's one thing to remember, remember that one. May vary from kid to kid though. Also, read this thread with lots of links for free printable materials to prepare beforehand.
His Own Travel Kit
A specific "travel kit" / "travel bag" where your kid decides what to take. Gets him involved and all happy to have his own "suitcase", to decide what to take, and to decide when to play with what.
Favorite Stuffed Pet, Blankie, Pacifier/Dummy, etc...
Never, ever forget one of these, obviously.
Kiddy Tunes and Stories on CDs.
You'll want to drive a knife through your eyeball after the 42th re-telling of any "whoever and the whatever" story, but he'll like it and be quiet. Except if he likes telling the story as well. In that case you're pretty screwed.
I prefer the ones that come with a book as well, so they can follow the story. Pro-tip: prefer rather long stories. I have a CD with 3 stories that are about 4 minutes each. After 2hours of listening to it, the knife alternative sounds entertaining. But if you have a 30 or 40 minute story, then it's not as repetitive and unnerving. You'll still have nervous twitches everytime they'll break into song even long after the road trip is over though. It stays with you.
Smartphone, tablets, etc...
Fine for playing videos as well, so if you have digitalized a few cartoons, you might want to transfer them to the phone, if he's old enough to not pause the video by mistake every 5 seconds (in which case the screaming reaction is probably counter-productive...).
Portable video players
These are actually fairly cheap nowadays. I never thought I'd buy into that sort of stuff and thought this was kind of a luxury for self-entitled kids and parents who can't figure out how to keep their kids busy, but... it does buy definitely you some quiet time, and makes them happy. Make them pick their DVDs before you leave (you don't want 15minutes of crying because you forgot the one they want).
Also relatively cheap now. They can be fun to have the kid take pictures of the scenery and feel like a road adventurer.
And similar things with lots of buttons and make lots of noise, and have lots of games. 3yo is a bit early, but that one is kind of a long-term investment you won't regret. We bought a cheap "notebook" for our kid when he was 1.5yo. Couldn't play the game, but liked to copy us and hear the sounds. Now at 4, and as the thing survived (surprisingly...) until now, he can play the games and still likes to take it with him. I'm not talking anything to technical or fancy, I mean a rather basic "computer"-like toy here. Something that just helps to learn the alphabet, has the kid count asteroids or find a number or letter, etc... Might be to advanced for him yet, but just the noise and copying you if you have computers at home will help.
Nourishments and Comfort Food
Don't forget to pack proper food, some snacks, some drinks, etc...
Stuff To Do
A few additional recommendations:
Breaks are mandatory. Unfortunately, breaks with kids always take way longer than you'd like to and they very significantly increase your travel time:
But they're worth it for your peace of mind and everybody's mood. Use the occasion to exercise, play with the kids, bathroom breaks, buy a super cheap toy so the novelty keeps them busy an extra hour or so (pro-tip: avoid toys that have red or blue lights... did that once during a night trip, and thought a police car was trailing me every time he'd turn it on... bad, BAD idea). Possibly even reload on cheap cartoon DVDs as well while you're there.
Of course, scheduling breaks is kind of dark art that requires a voodoo priest or something. It just never works right for everybody.
Make it Comfortable
Adjust the seat, bring a blanket, window shades, etc... Basically anything to make sure they're comfortable (especially if you adjust the air con for yourself in the front seats, which may feel very different in the back seats). Remember: a comfortable kid is a kid who has a greater chance of falling asleep :)
Novelty is on Your Side
So keep it alive. If you buy more than one new toy/book/movie, before or during the trip, don't show them all at once. Release them as you go. Keep the novelty effect on your side.
Not exactly ideal while driving, so keep it safe. But just counting (or pointing at, if he doesn't count yet) colored cars, trees, cows or whatever can keep him busy a while.
Drive at Night
Like Joe said, there are perks. Maybe not the whole trip though (it's difficult, and you have less service in rest stops if needed, and it's potentially more dangerous), but at least a part of it. There's kind of a difficult spot with some kids though, when they get tired and cranky but before they fall asleep. That, and also they might not be the only ones feeling sleepy.
Seriously, I don't think I've ever gone on a road trip where my significant other didn't nose dive during the first hour and then for a good portion of the trip. But usually so do the kids. :)
Check Your Car
Not exactly directly relevant to your question, but just as a cautious reminder. Because think of being stranded on the side of the road waiting for help to arrive. Now think of that same situation with kids who are bored, hungry, and when you run out of nappies, spare clothes, and it gets cold.
Again: a comfortable kid is a kid who falls asleep! :)
Of course, if you have another person who can sit in the back and play with the kid, your life gets considerably easier (theirs, however, that's a different story... I remember the mommy crying more than the kid, on occasion :)) and you don't have to resort to attention-grabbing things like DVDs/smartphones that much.