Pat M,
I think you can have all in a par 3. (Fear, toughness, Fair)
The challenge of, say feathering a 7-9 iron to the right pin at Augusta 12 is similar even if the penalty changes. If you can hit the target its fair. If you also have some kind of a bailout, its fair. If the right side was much shalower or smaller, or if the penalty was more severe, it would be unfair, and even more fearful. Its a great hole because of how those things balance.
Its fair because the left side of the green offers a bailout and even the prime target is deep enough and wide enough to give you a chance to hit it. Its borderline, which creates the fear. I think it can be as shallow as it is because the ball is on a tee and the distance is controlled. If it was a par 4, some folks might be coming from longer distances and/or the rough.
It also creates fear with the hazard in front but has a recoverable hazard in back. If that size target was a peninsula, making it like TPC 17, it wouldn't be fair unless the target was bigger, IMHO. If the green sloped away, or even too much to the front where a ball might spin back the slope, it would be a lot less fair.
In some ways, you can measure the fear (and fair) factor by how many people go for the Sunday pin vs. how many don't. If nearly zero % go for it, its probably not fair, or too fearful. If everyone goes for it, something is wrong, too. If about half try it, then you probably have the right balance.
I think Ross said it more succintly than JN or I - a par 3 can be a bit tougher because the ball is on the tee, giving the player a bit of an advantage.