X3's a flawed movie. It's a frustrating experience knowing the potential it had. The first film, like most of first entries in the comic book movie genre seems a little clunky at times. It's a bit dull, the fight scenes are badly choreographed, and it's low on action, but it's a pretty good movie. X2, I felt, blew it away. So I was really disappointed when they decided to go this route with X3. Singer abandoned it, or at least was unavailable for the date Fox wanted so we got stuck with a bland director, a standard by-the-numbers script, and ultimately a disappointing movie. I think i'll have to watch the movie again before I can really comment on the changes that should be made, but I really think it mostly lies with the script, and the portrayal of the characters. The film needed a good rewrite and God knows Fox wouldn't allow that if it meant delaying the shoot a bit. I think Singer was the main driving force behind the first two films. Fox didn't understand the property, heck it even seems like they loathed it. But they knew they could make money off of it and that's what they did. It's a shame, really.
Seeing all the potential that was missed, I actually wouldn't mind seeing a reboot a few years or decades down the line.