standard nut + spinnet nut + proper tightening = adequate protection against throw prop. The triple nut setup works, of course, but really not necessary.

If the backfire is strong enough to loosen a pair of nuts tightened against each other, it will surely loosen any of the two nuts in the triple setup. Think about it, you can have 20 jam nut in series, how does the last two nuts know they are butting up against another nut or the prop washer?

If you've taken Static Mechanics I and know how to draw a free body diragram, you can figure that out in a jiffy.