Currently in 4.4.8 and also 5.0, I guess, you can use mathematical expressions in values inputs. It’s good but you need to manually enter the canvas width and height if you want to compute a third.
What about adding an option to process the values as fraction ?
Now the grid applying the rule of third is easily displayed.
It is more or less a complement of another feature request Change Unit Type of Grid to mm, or inches etc.