While good, Jurassic Park III was a significant drop compared to the other two movies. There are three main reasons why I think this.
1. The battle between the Tyrannosaurus and the Spinosaurus. While kind of cool, it was too controversial. The fight itself was questionable and there are far more T-rex fans than Spino fans. It was, in my opinion, one thing to replace T-rex with a new dinosaur, but another to have it killed off by the new one. Everyone saw T-rex as the unstoppable king and then some weird sail-backed dinosaur comes in and kills the king? That was not Jurassic Park's finest moment.
2. The animatronics. Although they were ok, it was very obvious when it switched from an animatronic to the CGI. When the T-rex was stumbled upon for example. It's a minor detail, but I thought they did a better job with the animatronics in the first two movies.
3. The plot. The plot was not nearly as good as the first two. This is probably partly because the third movie was not based on a book like the other ones were. The dinosaurs were also more like Hollywood monsters than animals. It was like a never-ending chase scene.
While it had some flaws, some pretty major, Jurassic Park III was still a decent movie. A good movie in general, just not as good as the proceeding two.