You can put your sewage plant next to the upgraded water station and use filter pumps for unlimited water. Or up the water in the other two cities. When selling to other cities, the units are divided by total cities in the region. For example, if I have a city selling 100 units of something to the region, and theres 4 other cities, only 25 units will be available to each. This is good and bad.
The other two cities are already pegged on water pumping and barely sending 84? whatever (going from memory right now).
I'll try putting the sewage plant next to the water plant and see how it goes.
I also saw a YouTube video that says planting trees in ground polluted areas helps, so I've been doing that too.