I was thinking about it at work and I have an idea for it that may work. Have the 3 120g tanks have a 10g tank and a 20g tank. A water bridge connects them and a canister filter runs between the saltwater and brackish tank so as freshwater from the current comes in saltwater stabilizes the salinity. On the salt water tank it runs into a series of long thin pipes to evaporate and the water vapor goes through a pipe that branches off from that into a 20g tank and fills it up. Then once every so many houses a second pipe open and water is let through to flush out the pipe getting heated and drains into a 10g tank and the slat gets mixed back in. From there a pump pumps it back into the saltwater tank.
That's the best way I could think of. Not practical, efficient, or cost effective but if it could be pulled off that be incredible.
Keep calm and drum on