The modern wedding cake as we know it now would originate at the 1882 wedding of Prince Leopold, Duke of Albany; his wedding cake was the first to actually be completely edible. Pillars between cake tiers did not begin to appear until about 20 years later. The pillars were very poorly made from broomsticks covered in icing. The tiers represented prosperity and were a status symbol because only wealthy families could afford to include them in the cake. Prince Leopold's wedding cake was created in separate layers with very dense icing. When the icing would harden the tiers could be stacked, a groundbreaking innovation for wedding cakes at the time. Modern wedding cakes still use this method, with an added form of support with dowels imbedded in the cake to help carry the load especially of larger cakes.
Melt a little marmalade in the microwave or on the stove, until it runny. Brush this over the cake, it will stick the marzipan to the cake. I find the easiest way to get the layer of marzipan draped over the cake safely is to first drape it over a rolling pin, and then roll it over the surface of the cake. Dust it with some icing sugar or cornflour first to stop it sticking.
You should use a recipe specifically designed to be a wedding cake as it will ensure that it is sturdy enough and that it makes the right amount of batter and icing. For a three-tier cake, you need to make three cakes of 12", 9" and 6", as well as a massive amount of icing. Most three-tier cake recipes have a step-by-step schedule for when you will need to bake and assemble each part.