Each inner surface can support a ~10 mm layer of stationary air. This is what provides most of the insulation. So triple glazing will have four internal layers of air and will double the insulating value of the windows (assuming the units are 24 mm thicker). It doesn't need to be glass, there are some triple glazed units with a plastic film inner pane, they are lighter and thinner.
Triple glazing should allow bigger windows that still meet building regs thermal requirements.