This may seem like a stupid question but why would car racers need a 3 channels receiver. The cars only use 2 channels steering and ESC. so whats the 3rd one for?

Anything you want....some use it to switch to reverse (Nitro)....some rig up other things.... It's just there to allow you to control something.

I use the 3rd channel for my personal transponder.

I use it for my PT, and the 4th for RX battery.

Not much use for traditional scale electric racers, but:

Mechanical brakes with adjustable mixing off the throttle channel for a 1/8 scale electric conversion.
I’m not sure, but I think some 1/5 scale guys have separate front and rear brakes, adjust front/rear bias.
I think some may use it for adjusting nitro fuel settings.
Dig function on a rock crawler?
4 wheel steering monster truck.
Remote nitro starting.
Nitro reverse as mentioned.
Don’t some monster trucks have 2 speed trannies that you can shift with a servo?

OK, so not all are racing per se.

I use the 3rd channel and remote trigger cable to control my FlyCamOne2 mini digital camera for in-car video.