Hi and welcome sara...i'm still on my first run through GT3 myself. I havent actually completed the series you mention you're having trouble with. If you go to the section in this forum titled GT# Race reports, you'll see a thread there by a bloke called 'Uncle Harry'. He has a full run down on the entire game he completed. The thread is called "GT3 by Uncle Harry...0 - 100%".
I have found some pretty helpful stuff there. He covers the whole game.
With regard to your tuning problems, there is a site which has tune ups for just about every car in the game. You'll find some pretty good set ups in there, start by looking for those done by a fella called 'Fumes'. I have found that you won't go too far wrong with his settings. I've added the link to the site if that helps.
It's great to see that i'm not the only person out there who is only on their first run through the game, having picked it up at Christmas when we got the kids a PS2. The game was included in the package. I'm currently halfway through the Professional Leagues, have completed Beginner and Amateur, got 3 of the endurance races out of the way, and just picked up my rally license. Still to get my Super License.
Good luck with the series you mentioned.
Cheers
http://www.gtvault.com/gt3/index/
Just click on the GT Car List in the navigation box on the left, it will take you to the full listing. There are also tuning guides in the site as well.