[cessnadriver]

RICK. As an A&P and '65 C337 owner. The simple answer is YES, if there is an STC you'd have to use it OR DER field approval! Field approvals are long expensive exercise dealing with the FAA. The wiring would be fairly simple. I wired separate wires from each up-locks to three separate toggle switches on a test board, which had a molex connector, to test each up-lock. BUT this was only for maintenance checks, so it does apply to airworthiness requirements!
FYI: I too would like three indicator lights! If you find a STC, let me know!! BILLS