Isles are 31 inches apart, but seats are only ~17-18 inches wide. Stacking 3 tall gives you ~17.5 x 3 * 31 square inches. If you give 20 inches for bunk width that's 6'8" for bunk height which works out.
I don't think you could safely stack three of these, but it's going to be more comfortable on long flights.
I am more concerned with people exiting quickly in an emergency. It might work if everyone was a fit 20 something who practiced it. But, many people would have issues even in normal boarding, crawling over people with a carbon filled with smoke would be a real challenge.
I don't think you could safely stack three of these, but it's going to be more comfortable on long flights.