This can be done if a bit finicky. I would run a 20 gallon high as a sump. For ease of setup I would probably have 3 different pumps, one for each tank. Making an overflow for a 5 gallon and 15 gallon would be interesting but doable if you have some decent diy skills. Not sure if there are out of box overflows that will work for a 5 gallon or not. Also will want to make sure the sidewalls on the 5 and 15 are not tempered before drilling.