User blog comment:MaokiIveson/Disney cars racer number three?/@comment-5194340-20150127175000/@comment-3438245-20150127211835

Indeed, it seems Jeff Gorvette was supposed to be number three. It's likely they changed his number late in production and so they couldn't go back and change the numbers of the other racers accordingly.

That's for the real-world explanation... As for a in-universe explanation, the question is open, I guess: maybe Gorvette was unhappy to be #3 and made a last minute request to use his trademark number.