You cannot import your GT/GT2 licenses (or cash or cars) into GT3. You have to start with the 18,000 credits and the B-license like everyone else. The game saves are not interchangeable; since GT/GT2 saves can't be written to PS2 memory cards, and GT3 saves cannot be written to PSX cards. So don't throw out your old PSX cards if you want to play GT and GT2 again.
Yes, GT2 allowed you to skip the first two licenses if you had a GT (or known as GT1) game save with all three licenses completed. And GT4 allows you to do the same with your saved GT3 licenses. But GT3 does not have this feature, as I recall. So it's good practice for the new physics model.
If you have about 2-3 hours (and maybe some searching around in the GT3 sections), try to win your first car by getting all-golds in the B-license. This way you keep the 18K and get a prize car as well; it's not easy, but even I could do it.
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