I watched some of that, I wonder if the only difference between the 3" and 5" machines is the backing plate? The specs show the 5" weighing half a pound more, which seems like a lot of difference for just the backing plate. Was just musing whether you could just get an extra backing plate and swap them, rather than buying 2 machines.
EDIT: I see the 3" is 12mm orbit, while the 5" is 8mm...so they are different, and oddly so, I would expect those orbits to be reversed, but perhaps there is a method to the madness, like making the 3" a capable machine.