Anthony,
My answers to your questions:
--Would it have to be three? This is open to debate, but in my opinion, I'd say no, not necessarily - two or four (or more; one probably wouldn't work) suitably penal and suitably strategic bunkers would do the trick.
--It doesn't matter what the architect does or doesn't call the feature: if it functions as the Old Course's version of the Principal's Nose does, and one could reasonably demonstrate why in an intelligent conversation about it, then there's no reason not to call it a PN complex. (One can identify a Cape hole without it being called one, right?)
--I don't know Minnesota Valley at all, but I can't imagine any bunker complex behind a green being considered a PN. That's not at all how the feature works.
Ultimately, I'm guessing there are very few true PN complexes in golf, by my definition. None of the bunkers pictured in this thread would seem to fit the PN type to me: the first bunkers are to the left of the fairway and therefore not strategic; the second (at Yale), apart from their utter wimpishness in this incarnation as pictured
, shouldn't really come into play at all and are not strategic; even the bunkers at Chicago, a course with which I'm admittedly not familiar, wouldn't seem to fit the definition of a PN, as they again would seem to fail the strategy test. And frankly, I'm fine with there being few PN sets of bunkers - just because an ancient, revered and utterly great design feature isn't copied ad nauseum doesn't mean it's any less great for that.
Cheers,
Darren