Just a thought about this faux concern vis a vis 3 transponders. Uninformed complaints / criticisms seem to abound in the media these days.
1. The IFF/Transponder system doesn't need to be triple redundant, the way some of the flight safety systems mentioned above might need to.
Why?
The plane flies safely with 0, 1, or 2 operational.
If 0 are working, the crew need to use their radios more often in terms of coordinating with ATC on where they are and what they are doing, since the information isn't passed by the system via the radar and radar returns.
You can argue that if you wish to "for safety's sake" rely on a third layer of capability, that third layer is present in the ability to use two way radio communication:
You tell them where you are and what you are doing, just as we have always done in areas where there is no radar contact.
ATC then associates that with a radar reflection, rather than a transponder paint/code, and can thus track you and sequence you into the air traffic du jour. If you turn off your two transponders, and refuse to talk to them on the radio, it amounts to the same thing as turning off three transponders.
May we put to bed (without any supper!) this "three transponders" thing?
For any journalists reading (Mr Bailey?): please, give it a rest.