Have tryed numerous different ways to complete the challenge. Simply won’t pass, my guess is some sort of glitch stopping it. Finish 1st on expert, finish 2nd on high and finish 3rd average. Very frustrating when I’m sitting on 47% and only need 50 for the mustang 
Those aren’t race numbers. To get 1 star, you need to complete all races on Average or higher and end up in 3rd place or higher. To get 3 stars, you need to complete all races on Expert or higher and get 1st place.
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if you just win on expert you get the whole 3 stars
maybe try some of the other events to get higher than 50%, it’s not that hard