The "three wheels on the ground" trick works, provided you can keep it stable. If it wobbles back and forth, on and off that wheel, it will be slower, not faster.
Buy oversized nails/axles, and true them up in a lathe and polish them, watch the burrs under the heads. Get them big enough that once true, they are still oversized, so that you can do the same to the wheels. They sell a mandrel you can use to mount the wheels.
The really fast cars are the ones that have had the most attention paid to the wheels and axles being trued up, polished, and most of all, properly aligned. That makes more difference than the "tricks" and a car that has had that done, but no tricks, will be a trick car without the detail work 9 times out of 10.
We used to have "building days" where we got our scouts in the shop, and taught them to do stuff themselves, with supervision. We showed them how to build cars, and how to make them fast. And we taught them it was work, effort, and attention to detail that makes winners. The only things I did for the scouts were things that were too dangerous for beginners to do. And even then I made them help with all of the set up and details, they did the work right up until the power was turned on.
I did build a couple of my own, the adults would race their own. I had real narrow bodies (think old indy and dirt track roadsters), with dowels to hold the axles out so the track width was legal. I even streamlined the dowels. And I went so far as to drill the entire body out so I could put a round ball in it for ballast. I put a steel ball in for ballast that sat in a shallow recess in the back, and then rolled forward as the car went down the incline on the track. It hit the inside 1/2" back from the nose about the time the car hit the flat. I drilled holes in the wheels to lighten them (I also crowed them in the lathe, they're too wide), then used little "price dots" to streamline them (like Moon hubcaps) and hold dry graphite and moly.
The list of things to do is endless. But I always made the kids decide what to do, and made them do it. We had a lot of fun with it over the years.