[MovedGoalPosts]

From the IP of the OP I suspect he isn't UK based. Different standards may apply in his country.

In the UK running a cable up the outside of a block of flats would need consent of the other owners in the building. I'd be surprised if something like homeplug would work, even if the roof does have power, since three storeys away it's likely to be on a different power circuit / meter / phase.

I would presume you are using the laptop on battery power. You might just have to plump for a dongle to a mobile internet service.