X-Men: The Last Stand is the worst of those three if you consider their respective franchises.
Spider-man 2 is one of the most over-rated superhero movies there is and Spider-man 3 gets an unnecessarily bad rap as a result.
Blade Trinity is more of a comic book movie than Blade 2, which despite being a cracking film, i think steered expectations and non-marvel readers' opinions are in part responsible for criticisms of the third film.
That leaves X-Men The Last Stand, which, though i enjoyed it, like the rest of Fox's X-Men films, is messy, and wastes characters by throwing them in for the sake of using the name without fleshing them out, and lowered the bar for future X-Men movies which continued to disrespect the original text.
Wow ! That's one of the most insightful and rational comments I've read in ages.
My take is a little different.
Blade Trinity: personally I find the Blade films take themselves so seriously to the point of being a bit ridiculous - especially Wesley Snipe's one dimensional performance, which is sad because prior to Blade I really rated him as an actor. My other gripe with the Blade films is that ( until he fights the big bad at the end) Blade never really seems to be seriously challenged at all - and while Snipes martial arts talents are impressive, there's only so much post kill posing I can handle.
Blade Trinity is actually my favourite film of the trilogy, mostly because Ryan Reynolds Deadpool ( let's face it,he's auditioning for Deadpool in that film) patter takes the edge of Snipes utter seriousness and because Jessica Biel is just so badass in that movie and looks great while being badass. Not deep reasons to like a film,but they worked for me.
Spider Man 3: I too find Spider Man 2 to be massively overrated, but I understand why people like it so much.
Spider Man 3 has a lot of crap in it, from Thomas Haden Church's one note performance as Sandman, Kirsten Dunst's awful singing to the terrible miscasting of Topher Grace, and of course the nightclub dance scene.....just dreadful. To me the worst thing about it is the very last scene because it dilutes all the excitement and tension of the previous scenes and the emotion of the funeral - really they needed a Peter swinging off into the sunset scene.,
However it has a bunch of redeeming features including
1)The Harry vs Peter plot was really well done loved the fight scenes, especially at Harry's apartment ( when Harry says ".....strawberries." you just want Pete to kick his ass). Also, it has a very satisfying resolution which makes sense in the wider context of the trilogy.
2) The Venom creature, looks like it crawled straight out of the comics and that they included the bell tower scene, and the way it was eventually destroyed - nice nods to the source material.
3) Evil Disco Peter Parker. I found his strut down 5th Avenue montage just hilarious, I was nearly on the floor. After that he went too far with the emo look but up to that point he was great.
Overall, I found it disappointing but not an awful film at all
X-Men Last Stand:
Okay, the biggest sin here is the complete mishandling of the Dark Phoenix storyline, which is IMO the greatest Xmen story ever, if not one of the great comic book stories ever. I can't believe that someone else actually managed to do a worse job with a bigger budget and better cgi tech.
I liked Xavier's death scene, a real Obi Wan moment. A lot of characters were shoehorned into the film or simply erased ( like Cyclops, ugh)- although Ellen Page manages to be cool as Kitty despite only having limited screen time and the issues with Ratner ( very bad indeed) but that's because she's an amazing actress.
McKellen and Stewart were brilliant as always, a real legendary pairing.
All in all I enjoyed it in the cinema and have rewatched it a few times.
So which of the three is the worst ?
That's a really hard question. Compared to the other films in their franchises I liked Trinity the best, but it's a worse movie than Last Stand.
I'll go with Spider Man 3, as it's the least likely of the three films that I'd watch given the choice.