How do these three lenses perform in producing sharp images when used wide open?

It's often said in that the 50mm f/1.2 lens is Nikon's sharpest 50mm f/2 lens. At f/2, it's tack sharp, sharper at f/2 than either the 1.4 or 1.8.

Here's the thing: almost no lens is at its sharpest wide open. Every lens is different, but usually it's at least a couple stops less than wide open where the lens sharpens up considerably.

Here's the other thing: you probably will be spending a lot less time wide open on any of the lenses than you think you will at the moment. Sure, there will be plenty of opportunity to use them wide open, and at the beginning, you will probably be overusing f/1.2, f/1.4, or f/1.8 as the case may be. You will also probably be throwing away a lot of those shots, or working them extra hard in post processing, trying to figure out why many of those shots are decent but not quite right. That is, until you stop down a bit, and enjoy the sharpness and flexibility that f/5.6 - f/8 brings you.

But, how about the quality of bokeh? Is the difference at all perceivable in bare eyes?

Neil van Niekirk compares the f/1.4G and f/1.8G, and shows that their bokeh is very similar below 5.6 (and doesn't go past that; his review is focusing on bokeh). He went with the f/1.4 for himself, but doesn't have a strong recommendation for it over f/1.8, especially considering the price difference. (Personal disclosure: I own the f/1.8G, have used the f/1.4G. I like the 1.8 better). In his review, the bokeh between the two was not really perceptible, especially if you're not pixel peeping.

The f/1.2 is an older design. While it has great bokeh wide open, it also demonstrates coma and spherical aberrations. Depending on your subject and composition, those might never be an issue for you. Because of its manual focus, you will however miss a lot of shots, with a hefty price difference as well.