I am doing a three layer graffiti cake. The only three layer I have done before this, the top was a ball cake, and the total cakes (I use box mixes) were four cakes -- two for the bottom but one layer, and one each for the top two with frosting in the middle of each. I stacked those one on top of another and had no problems.

This cake I wanted thicker layers so I ended up doing three cake mixes for the 12 inch round (with frosting in between each layer). The middle tier is one mix in two 9 inch rounds, but I split it in four and put frosting and jam in between each layer The top layer is 6 inch round and approx 3/4 of a box mix (the rest didn't fit or was cut off to make it flat)-- so five boxes of cake total! .

All three layers are covered in fondant and alread partially decorated. If I just stack them one on top of another, will they be too heavy? I am worried the cake will collapse or the fondant will crack. A little cracking is okay -- is supposed to look "distressed," but I don't want a disaster. Is there more I should know about how to do this? I have never done supports or dowels or anything like that before. Also, the layers are in the fridge. Should I let the fondant warm up before stacking or not?

You will have to use dowel rods in the layers to hold the weight of the cake on top of it. Here is a link to give you an idea how to stack them.
http://www.youtube.com/watch?v=dwnKzv7PUso&feature=related