It appears that LittleBigPlanet 3 may be in the works at the British-based studio, Sumo Digital.
According to VG247, Sony and Media Molecule asked the developer to build a third proper entry in the franchise, which apparently has now been in the works for “well over a year.” Could Sony be hoping to get this out in time for the launch of the PlayStation 4? It sure is a possibility.
Media Molecule has made it clear that they wish to pursue new projects (like Tearaway) so reports regarding their departure from the series shouldn't come as much of a shock. It is also worth noting that Sumo worked on the LBP2's cross-controller patch, so this wouldn't be their first time working on the series.
Sony has yet to confirm the validity of these reports, so bear in mind that this is nothing more than a rumor for the time being.