I understand what you want to do and why, I just drilled my 13 gallon Vittle Vault and installed the grommet for the Airlock. I would offer caution with stacking them myself, due to a couple reasons. 1.) Like mentioned prior by someone else, the heat. I figure if one can produce 5 degrees while fermenting, then what will 3 do on top of each other, and the higher the temp while fermenting, the more liklihood of off flavors from fermenting at too high of a temp for the yeast. 2.) I know my fermenters (Plastic Barrels), when I lift them, the bottom sags under the weight of the wort, this causes the airlocks to inhale a bunch of air and liquid through the airlock into my sanitary wort (now beer). I imagine lifting one off the other, setting one back on the other, etc.... will create a kind of inhaling & exhaling and maybe suck in some nasties eventually. I might be wrong, but something for you to think about (stacking I mean).