David Vonderhaar has stated on Twitter that Black Ops 3 stats will not transfer between last-gen platforms (PS3/Xbox 360) and current-gen platforms (PS4/Xbox One).
— David Vonderhaar (@DavidVonderhaar) September 7, 2015
For players that plan on buying Black Ops 3 for last-gen, your stats will not be available when or if you decide to upgrade to a current-gen platform. For both Call of Duty: Ghosts and Call of Duty: Advanced Warfare, Activision allowed players to have their multiplayer stats available for use within the same console family (i.e: stats transferred from PS3 to PS4, Xbox 360 to Xbox One). It looks like Black Ops 3 will not support that ability.
Activision has also not yet announced any plans regarding the possibility of upgrade programs from last-gen to current-gen. We’ll update if or when they do.