The missus and I just saw Toy Story 3 in 3D and loved it. Like Up, I don't think 3d is essential, but I think you're missing out if you don't see it in 3d.
I'd imagine the short beforehand would be pretty lifeless and flat in 2d? It certainly wasn't the funniest of the pre films shorts, but visually was outstanding.
As for Toy Story 3, I think it's the best of the 3 (we watched 1 and 2 again last couple of weeks), as as well as being extremely funny, is very exciting as well
10/10