Can these filters be stacked or must they be used singly?
Yes, you can stack all three to make a 9 stop ND filter or stack two for a 5, 6, or 7 stop ND depending on the combo used. These are not the super slim filters so you'll have some vignetting when stacking on wider angle lenses. I haven't found vignetting to be much of a problem and if it's noticeable I just zoom in a little or change lenses and reevaluate my composition. Another tip is to focus and compose your shot before stacking the filters as neither you nor the auto-focus can functionally see anything with the filters stacked.