I saw the 2D-only version and there wasn't anything particularly jittery about it. It had the usual special effects issues with physics of unrealistic things looking, well, unrealistic (especially apparent in the dragon attack and the long shots when everyone is trying to run out of the troll mountain), but I didn't notice anything amiss that I could attribute to the 48fps->24fps down-conversion.

Either the 3D/24-fps combo makes things weird or JungleGymSam saw the movie in a place with a janky projector.