At their base, there's about 1/2" of space on either side when perfectly centered -- so if you wanted to build some kind of platform, you could still use 1/2" walls at the base of the cases.
View when stacked is just a hair higher than the side windows:
Update: Of note, the large versions won't fit 3 as they are each several inches wider. The small versions are 3/4" wider than the mediums, so I don't
think they will fit 3 across.