Have you checked your layout for any loose connections? Loose connections can cause surges. What make is your DCC controller and how much voltage is it outputting? If the controller is outputting too much voltage it could be the cause of your blown decoders. I suggest that you check out your whole system including the programming track, if you have one, until you find the cause of the problem. After 3 blown decoders I be very dubious about running any loco until I have had the system checked out..