Well I finally did a 3 in 2 days viewing (best I can do, I cannot sit watching TV, particularly the same movie story for 6 1/2 hours straight).
My thoughts:
The movies works well as a trilogy. Mostly from the angle that it is about these three dumb kids who learn about life in a superhero environment. Peter/Harry/MJ make these movies as a triogy have any depth that they would otherwise be lacking. They are dealing with the sins of their "fathers" (Peter's guilt over failing to live up to Uncle Ben's ideals, Harry failing to live up to scumy Norman's ideals and MJ just having a **** father that hurt her young adult life treamendously).
Watching them go from the hope-filled childish teens at the beginning of the first movie to the ultra-close depressed and worn grown ups at the end of the third movie is actually really well done. That thorough line is fun to see. And that is why GG being the first villain, as Harry's father makes sense and how he haunts EVERY character in the series (even Ock, as he was employed by Harry in this rush job to outdo Norman) throughout all three movies works. Ock is the most visually amazing and well performed by Molina, albeit he may seem the most remotely connected of the three, he is in the best movie and rocked the world. Venom even makes sense of being the very last villain because the whole trilogy Peter is learning how to grow up and be a good man and Eddie is everything Peter could have been without Uncle Ben's teachings, comittment to MJ or Aunt May and a noble conscious, etc. etc. Venom being the last defeated worked in this context.
As individual films though, 1 has a childlike wonder and fn level similar to S'78 and is a lot of fun. SM2 is still hands down the best one as it is just so well executed. The acting, the story, the direction, the pacing and the very smart screenplay all work to a flawless ending.
SM3 is both helped and hurt in view of this context. It's flaws compared to the first two movies (too many characters, too much subplot, the underdevelopment of Sandman and Venom and ccertain plot holes like MJ caving on Peter at the first sign of risk) are more readily apparent than ever. But at the same time it is strengthened as seen as act III of this over-arcing story and makes the ending of the movie more satisfying. It also makes any scene involving Peter's relationship with MJ, Harry or MJ and Harry feel more natural and provides a more satisfying (if downbeat) ending.
So in the end I think it hurts and helps SM3, but as a trilogy it is quite strong. Moreso than Pirates or X, anyway. But Star Wars, it ain't.
My 2 cents.