I don't get this with the "hard drives (plural) from the machine" as any MacBook Pro from the 2016 models (both 13" and 15") only had space for one hard drive (SSD).

Models preceding those had space for an optional DVD drive that could be converted for an extra hard drive, but that was a conversion involving installing non-Apple kit.

If we are talking newer kit, Apple has soldered the SSD to the man logic board making it impossible to replace it as the T2 chip that is the controller for the SSD is paired with the drive logic and will refuse to work with a replacement.

A repair shop might be able to image the content of the drive given the T2 + SSD still worked.

So in reality there must have been 3 machines if there are 3 hard drives.