The properties of the asymptotic AdS3 space–times representing flat domain walls (DW's) solutions of the new massive 3D gravity with scalar matter are studied. Our analysis is based on first order BPS-like equations involving an appropriate superpotential. The Brown–York boundary stress-tensor is used for the calculation of DW's tensions as well as of the CFT2 central charges. The holographic renormalization group flows and the phase transitions in specific deformed CFT2 dual to 3D massive gravity model with quadratic superpotential are discussed.