Update for people that come here looking for a guide how to replace it.
The corners CAN be replaced. You can found it on ebay just searching "Sony xperia z3 corners" They are attached by presure clip to the main frame, you need to remove all the rest parts of the old one and clip the new one. Originally it goes thru a side, but you can put it by presure. Some chinese sell a model with glue to make it easier. BUT I don't believe that will last long.