Provided the legal requirement of available adequate spare correction is at least met, individuals are of course free to take whatever extra precautions they consider appropriate.
I’m guessing you don’t take the third set to your medical examination? I’m also guessing that you consider yourself perfectly capable of determining whether the third set is adequate by, for example, using this complex procedure:
1. Don spare vision correction.
2. Confirm that you can see everything you need to see, properly.
If that’s what you’re doing, are you sure you’re not being irresponsibly dangerous?
If your second set is only adequate if it’s been confirmed adequate during your medical examination, as required by AVMED, it follows that your third set is not an adequate backup in the event of failure of the first two sets, unless the third set has been confirmed adequate during your medical examination. This is the inexorable conclusion of AVMED’s reasoning.