I'm having A- Z same problem .. After replacing my z3 screen , back camera front camera and flash not working anymore.. Also flashlight toggle disappeared from home screen .. I think problem is related to flashlight . When I connect my back camera the phone heats up and battery drains by 1 % every 2 minutes ..But when back camera is removed battery drains by 1 % every 2 hours ..
I tried every thing from factory reset , repairing through sony pc companion , tried other camera apps , changed camera sensor etc .. I came into point there is some short near camera or problem is due to flash (As flash light is also not working and flash toggle has disappeard from home screen .
Also please note on diagonistic front camera shows up but back camera doesnot shows up .. In camera app when clicked I get a message " Unfortunated camera has stopped " .