While the .pkg file represents an easy way to add a "newer" title to a legacy console library, Call of Duty: Black Ops 3 on PS3 is widely considered a cautionary tale of cross-gen porting. It is functional, but it is a shadow of the intended experience. For collectors or those strictly limited to PS3 hardware, it offers a glimpse of the game's mechanics, but for the genuine experience, the PS4 or PC versions remain the definitive standard.
Copy the .pkg file into the root of your USB drive (e.g., USB:/BO3.pkg ) or the packages folder on your internal HDD.