I purchased the premium pack the other week during the black Friday sale and i was able to use the car then, but the other day i loaded up the game and the car was gone, the file for it is still there but there's no option for it in game?
The FR3 car has not officially released yet. What you experienced was most likely a bug that was quickly corrected. It is not included in the premium pack yet, as it has not been released. Sorry mate.