I didn't like T3 because of the whole 'can't change the future' thing. Both because I don't believe in fate anyway, and secondly, if changing the Future was impossible, how come this was the third Terminator that Skynet had sent back to do exactly that? And the Terminator that figured this out was of a make so old that they were obsolete. And, of course, if the future is immutable why did Connor bother sending back a Terminator to protect himself? If the future is unchangeable then the simple fact that Connor is alive proves that the attempt can never succeed.
It wasn't a bad movie as such, but it still left a sour taste in my mouth after the first two.
Personally, I think I'd prefer a series based on the resistance after the War to another film, do away with the whole 'Terminators are people too' crud and turn them back into the T1 mindless killing machine mode.