They are actually on various tenants electric circuits, there is no "house" circuit as of yet. The 1st floor gets the basement lighting, the 2nd gets the back hallways, the 3rd the front hallways....but they are all full aware of this and realized it prior to renting the apartments. I always strive to save them a few bucks when I can, but the lighting really is pennies when you come down to it, they get the benefit of the best landlord in the world for it
I am planning out how to rewire the entire house along with adding a 4th unit to the basement in the process. I have to deal with brick fire stops though....ugh I am dreading it.