It depends on the dimensions of the art, and if the sconces are in a position where someone at a gathering at your house could graze one of them with their shoulder. There are no standards for this other than what looks good and is practical.

I need to see the sconces and also the area where they will go but IMO a bit low . I actually do not usually like sconces so alot also depends on your styleWhat are you putting on the floor that will be 3" thick. A bit more info please

Depends on: art’s size and therefore height, style of sconces, placement in middle or upper half, allowing for a piece of furniture under art if need be, which would change art’s placement.

I have three 11" Blade LED Wall Sconces two of which are between windows in a room with 8' ceilings. Bottoms of sconces are 65" from the floor and centers of the 4" electrical boxes to which the sconces are attached are 67" from the floor. Lowering the electrical boxes by 4" (to 63") would have made them too low, I believe.