How do you stack a three layer wedding cake?

Normally with the biggest layer on the bottom and the smallest layer on the top.

If you're asking how you keep it from collapsing in on itself due to the weight of the upper layers...

Caterers generally use clean wooden dowels, about the diameter of a standard pencil, cut to the height of the cake. These are inserted into the bottom (and middle) layers to support a piece of thin plastic that the next layer sits on. You don't need many... maybe one in the middle, and another four in a circle just slightly smaller than the plastic piece.