hey jeannie, i'm not a confectionner but i'm guessing the icing on each layer of cake is complete, that is you ice the cakes before you stack'em. usually, the smallest cake or the top tier is reserved for immediate family and the bottom two tier are served.
if your cake is a heavier cake, that is not a sponge cake, then you have to use 'pillars' to tier the layering. otherwise, if it is of a heavier consistency, like a dense fruit cake, they rest ontop of each other.
to serve, the disassemble the cakes and cut them as you would normally.