As a fan of the series, my reaction to this was lukewarm.
First of all.. the graphics are disappointing.
The "sackboy friends" don't really bring anything new, all of their abilities are pretty much already present in LBP2 either through power ups or customized sackbots. It's nice to get "official" wall jump controls and such but it's nothing groundbreaking. And the "air pump" gun could be replaced by the creatinator in LBP2 at its default "shoot water" setting and that would fulfil the same role. Not sure why that power up is special, yet, or why they chose to show THAT at E3 of all new things.
The "added depth" element is more like it, but it looks like something that will only ever be half-there and rarely used properly, and mostly for mini games (see: the tennis minigame in the trailer) In other words, it is just like the addition of top-down level design in LBP2. 98% of levels will still be sidescrollers, simply because that is still going to be the easiest thing to make, and the new plane will get the most use through mind-blowing glitches.
But if LBP3 just succeeds in being a less constrained LittleBigPlanet 2 and gives us significantly more level memory, I'll probably get it.
Nah, it's obvious that Media Molecule is working on something with 3D and it's smart not to have both of Sony's "create, play, share" games being the same thing, I will probably end up getting both, purely for the fact LittleBigPlanet 3 will have 2D gameplay elements that Media Molecule's new project might eschew for a core 3D experience.