Gran Turismo 3's main goal was to go for ultimate realism, and I'd have to say it wasn't far off from that. Sure there have been new and bettered installments to the franchise, but this one really outdid it's expectations in realism. The only thing where it fails in comparison to GT2 is the amount of cars. That doesn't mean there's not plenty to choose from, and you also get to tune them in detail. There's so much to do in this game, even if it's a bit repetitive, it's still fun and addicting.
I remember back when I got this game, I entered one of those endurance races to get easy money because even last place was rewarded. So I managed to finish in fourth place with my fifth(?) car in the game. After that I bought a Mustang and did the same 1 hour race and finished in 1st place and got the Polyphony F1. With that car the rest of the races, or at least the ones you could participate in with that car, were extremely easy. Good times, but I wish I hadn't gotten that car so early on, because it kind of ruined it for me, even though it was fun to beat all the opponents by several laps.