VG247 reports that sources points Little Big Planet 3 being in development at Sheffield based Sumo Digital. With Media Molecule busy working on Tearaway, it is reasonable to think Sony would want another developer to work on Little Big Planet 3 in the meantime. No word on what platform it is in development for, it could either one of the latter big games on Playstation 3 or an important early game on Playstation 4.
Sumo Digital have previously been involved with the franchise as they produced the cross-controller DLC for Little Big Planet 2.