IMO there isn't really a "Third movie curse." It's more of a "weak link" curse that seems to affect a lot of trilogies, and in many cases it is the third movie. In a case like the Matrix, we saw it go from awesome, pretty good, and then crap. However, while the third movie is often the weak link, there are plenty of examples where it's not, or at least not enough to be a major detraction to the franchise.
With Terminator 3, it was an okay movie, but it just wasn't in the same league as the first two. I wasn't dissapointed because I knew not to go into it with high expectations, but it's still one of the lesser examples of movies effected by the so called "curse". Jurassic Park 3, and to a lesser extent, Star Trek 3 would fall into this category as well.
Return of the Jedi was a great conclusion to the Star Wars trilogy, but it wasn't as good as Empire Strikes Back, even though in some aspects it was better than A New Hope. I put X-Men 3 in this category as well... It wasn't as good as the second one, but it still outdid the first in many ways (although it loses some cred for not being tasked with establishing the franchise).
Then, there's trilogies where one of the first two is kind of the oddball, but then with 3 they returned to full glory. Examples of this would be the Star Wars prequels, Indiana Jones, and Back to the Future. Episode I just plain wasn't that hot, but Episode II was a lot better, and with Episode III, they'd finally just about gone back to he bar set with Empire Strikes Back. With BTTF and IJ, the first was great, the second was pretty good but kind of strange, and then the third was great again.
Finally, there's the most prestigious category-- Return of the King. Return of the King actually managed to surpass both of it's predicessors. Return of the King truly is the best movie in the trilogy IMO, and that's rare. Unlike Revenge of the Sith, it wasn't just reclaiming glory that was lost by a previous weak link-- it was building on what was already the makings of an excellent series. I can't think of another trilogy that pulled off it's #3 as well as LOTR did.
So that leaves the question-- where does Spidey 3 land? Personally, I'd put it in the "Return of the Jedi" category, although I'd say it definately ranks at the top of it. I'd put it like this:
1. Spider-Man 3
2. Return of the Jedi
3. X-Men: The Last Stand
Spider-Man 3 was an excellent sendoff for Raimi's trilogy, and while it wasn't as good as the second one, it's still on par with the series expectations for the most part, and it's definately a solid bookend to a great series.