The only unnecessary part was seeing it in 3D, but that's a separate issue entirely.
I didn't think another sequel was absolutely necessary; I would've been fine not knowing exactly what was going to happen to the toys after Andy "grew up." It was still worth seeing, though, and I'm glad they revisited the story. I was really, really excited about seeing it (and saw it today) because I loved the original so much and thought the second was pretty good, too.
In short, I liked it. It was a lot like the prior two; the entire time I was really, really anxious, even though I knew all the toys would somehow end up all together and safe in the end, and I laughed a lot. And then I almost cried a little at the end, which I will admit even if it makes me sound like a cheeseball.
In my opinion, it wasn't bad at all.
Oh. But they didn't leave in the part with the dinosaur and the IM with the other dinosaur down the street (which was in the trailer). I thought that was really funny. Also, that old teddy bear thing/guy, the one that smells like strawberries, reminded me too much of the Prospector from Toy Story 2, so that was just a bit generally weird.