I would suspect that there is a high impeadence line to ground connection in the vacinity. This is allowing a low level of current to flow through the cold water pipes into the earth then return through the neutral ground link of the originating service for the offending circuit.
The damage to the pipes is due to the galvanic action from the charged pipe to the minerals in the soil. The pipes are being slowly eaten away. Induced current cathodic protection systems seek to protect underground pipes in the reverse.
The high impeadence line to ground circuit could be in a 240 volt dryer that needs a neutral, but is connected to ground instead.
If you clamp an amp meter around the ground electrode conductor in the three homes you might be lucky and find the culprit. Max Billington
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