I would close all three. I would remove the railing and all the molding expect for crown molding then sheet rock them. Paint the new wall the same color as your current wall.

I’d close up all 3, flat and even with the rest of the wall. And then I’d spend the next 12 months fighting the urge to hang anything on the newly flat and even wall because it’s going to look like the largest empty expanse you’ve ever seen, and it will take 12 months to get used to the change and forget about it.

Just an idea if the two on the right only were to get closed off. For stairway you could simplify what gets hung there, perhaps a vertical row of 3-4 medium sized framed prints.