If the core element of the movie is good then eveything else is irrelevant. Not saying that SM3 was great but I see it as a good movie with bad scenes rather than a bad movie with good moments if that makes sense.
The core element was very bad. My biggest problems are the characterisations. I hated what they did to Peter. The only part they got right was he's a nice guy. Honestly this isn't the charming, confident, smart but also a little awkward College years Peter from the comics. He's even more shy and reclusive than in high school. About Spider-Man he didn't make much quips but I liked the two we really got. Mary Jane was terrible, she was selfish and was really winy. I was cheering on Venom in the finale.
Now Sandman was actually great, unfortunatly he killed Uncle Ben by accident. It's not that I don't like the idea of it as I am not a silly purist but it changed the meaning of Ben's death. In every version, Peter is responsible as he let the thief get away. His killer murders him intentionally. In this Uncle Ben's death was a total accident, Marko got spooked and oops I shot some old guy. Plus your man in the first movie died for no real reason.
Pointless characters like Gwen, I don't blame Raimi but I didn't think they emphasised the strong female character she was in the comics. Now they improved Venom's motivations but they left out his Cancer and his Wife! Topher Grace tried his best as Venom but he just was miscast IMO. Emo Peter Parker, I'll leave that out!
What do I think of Spidey 3? It's pretty bad but ot awful, that's reserved for ones like Ghost Rider or Green Lantern. There was positive qualitys, action, score, Franco's acting, Sandman birth scene, the theme of good and evil blured. If they fixed the script for the leads and Sandman it would have went along way for me liking it.