Activision has announced today that COD: BO3 will also be coming to the previous generation consoles, since a lot of gamers haven't updated their consoles to the current generation.
Black Ops 3 was built from the ground up to harness the full potential of the Xbox One and PlayStation 4. To ensure that the games development won't be hindered in any way, the developers at Beenox and Mercenary Technology will be porting BO3 to the Xbox 360 and PS3. This leaves Treyarch to work solely on the Xbox One, PS4 and PC versions.
Unfortunately the previous generation consoles will be missing some features due to the power difference between the consoles. For example, the singleplayer co-op will only allow two players compared to four on the Xbox One and PS4. There is no Nintendo Wii U version being planned at this time.