The variety of cars in GT4 was borderline amazing and insane (duplicates! duplicate! duplicate '91-edition!), but it felt a lot more like work at times; since there were a lot of different varieties of races, there were plenty of other depth and areas of the game to have fun with, even if you didn't care for Missions and the like. FF cars handled like...well, like most FF cars. I think they could have made that part a little less realistic in the name of "fun". I'm still not sure what the point of B-spec was, other than to add a pointless aspect to the game...why not just have horse racing, or any other parimutuels? Today's Special wheels was my favorite non-racing parts of the game, kind of wish it was in GT5. Prize cars weren't much of a selection - take it or leave it.
GT3 had an interesting flow to it, the prize cars could either be tremendously rewarding or an extended middle finger, and the inconsistent AI was obnoxious at times. I felt like too many of the cars handled too similarly from one to another - with obvious exceptions - it seemed like the initial handling characteristics of many GT3 cars seemed straight out of the original GT, compared to even GT2 and GT4. The races didn't seem long, but it sure seemed like slow-going getting out of the Beginner Series. Also, silly pit scheduling by the AI. If I'm not mistaken, the so-called "emotion" physics would hit you back in retaliation if you didn't make room on the first pass, something it didn't do in any other games in the series.
All in all, you could live without GT3, and wouldn't miss much. But it definitely wasn't bad.
Edit: Just realized our so-called GT BOSS (the OP) should have known these differences, but he's GT Banned.