How many units are in this development? If its only three I can see why its unclear but the owners should be able to read the documents they signed when they purchased the condo that spells out who is responsible for what. If this is a larger development the homeowners association should be able to settle this. Typically individual owners are responsible for anything inside their home but anything common or on the exterior is the responsibility of the homeowners association. If this development only has 3 units then each owner will be responsible for 1/3 of the cost. I don't thing the electrical inspector is going to be of much help since this is a legal matter.