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When Vanellope is being taught how to drive Ralph is guessing the functions of the 3 pedals in her car. His guesses were:

• Right: Accelerator


• Middle: Brake


• Left: Useless

But when Vanellope is driving by herself after being taught, we see she's only using the Right and Middle pedals and she seems to only use the left pedal when she shifts gears (suggesting it's the brakes?)

So what are all 3 pedals for in Vanellope's car? Or did one get added by mistake?

Someone doesn't drive stick...:)

The third pedal is the foot clutch and is only used when changing gears in a manual transmission automobile.

A clutch is two metal plates in the engine. When you press the clutch pedal down the plates come apart separating the engine from the drive wheels allowing you to change gear. Bringing the pedal back up re-engages the plates which in turn connect the engine to the drive wheels.

^Quora

Also - http://www.drivinghelp.com/Pages/Controls_Pedals

Because Vanellope is from a car racing game, where, unsurprisingly, they drive race cars.

High performance cars classically used manual transmissions, because the human can make better decisions about when to shift gears than a computer can*, and the difference is plenty enough to be the deciding factor in winning races.

You may know that engines perform best in a certain range of engine speed, and that is not proportional to the road speeds. That is why cars have a number of gears like a bicycle. That's not the pedal, that's the stick between the seats. To match up engine speed to the gear you are changing into, automatics handle that with a hydraulic coupling... but on a manual, there's a gadget that allows thedriver to select a certain amount of "slip" between engine and transmission. That's what the left pedal does.

* least of all a hydraulic computer as would be in a non-computerized performance car, such as the THM350 or 700R4 loved by hotrodders who want automatics.

Cool 80’s story: There were two truly excellent brands of automatic transmission, ZF and Hydra-Matic. The latter is General Motors, yes really - any random Chevy got a world-class automatic. Once Rolls-Royce gave a trial to the Hydra-Matic. It ran fantastic, exactly what they wanted. But they did a tear-down and found a bunch of rough castings inside the hydraulic computer (valve body). Fine for a Chevy, but hardly up to Rolls standards! So they painstakingly cleaned up all the rough casting ripples and marks and made it look fantastic. Reassembled it, and suddenly it shifted badly. The mechanics were losing their minds, what did they do wrong reassembling it? They asked GM. GM said "You smoothed the valve body!? We deliberately leave the casting rough to crate turbulence to the hydraulic flow. No wonder."