All you have to do when you reach rank 3 it's to go to the dump and search for Platinium parts. And after repair it in the garage with the button x to obtain maximum level of the part. Your car must be equiped with only Platinium parts at the maximum level. You also need to match the parts in each categories with the same letter, like A, B, C or D. After this you must tun your car to increase performances, chose the right settings.
Don't waste time in challenging races to win platinums parts, I Always have parts tier 2 like that. You need Platinium parts tier 3. The only way is to go to the dump, it's totally random and you have to play again and again to finaly get the parts you need.
Hope this help