I've compete all challenges and career, used the trainer to enable all the cards, but I don't see a GT3 available anywhere. I do have the other cars, like the "custom" Audi Lemans, "bonus" Carerra GT and BMW M3 GTR.
its in the online rewards...
c in the bottom of the reward cards screen ...
me 2 in the same condition ...
can't use the net so waitin for the 19 megatrainer ...