This project is really interesting, but it requires a certain amount of organization and technical expertise. Installing directional antennas, running wires, etc. isn't incredibly hard, but it's a lot more work than setting up a router.
The new WiFi mesh routers are pretty great at covering a larger area and in my city the apartments are pretty close together. In a 3-unit building, it seems like you could just get one internet connection in the middle unit and a nice 3-unit WiFi mesh system and save on internet costs.