What's the best way to stack a three-tiered wedding cake that is not covered in fondant/marzipan?
The issue of fondant versus buttercream is one of convenience.  Fondant is (relatively) sturdy, pretty, and can be held for long periods.  In a wedding cake, which might be prepared and decorated over several days these are advantages.   True buttercream does not hold very well (it is at its peak only for a day or so) and is much softer, and it requires refrigeration, so it is logistically more challenging.  

Other than that, the fondant versus buttercream issue is not really relevent to the structure of your tiered cake.

Typically, each layer would be on a cake round (or in your case, square), supported by dowels through the layer below.  These dowels can be any food-safe material such as plastic or wood.  The cake rounds (typically coated stiff cardboard) are essential to the structure, as they provide direct support to each layer, while the dowels serve as structure to hold the round up.

The Wilton site has a pretty good illustration of the method.