Sometimes the third channel has been used for controlling winches in scale vehicles and shifting into forward/reverse for some nitro Traxxas trucks. All of my cars have three channel radios so that I can plug in my transponders and also because most "upgrade" radios nowadays are three channels anyways.

I would also recommend Radiolink for good, affordable radios. Their RC4G radio is very nice and extra receivers don't cost an arm and a leg. It's what I race with.