Quick Help with Buttercream Amounts

Hey Everyone.  I am taking on a pretty big cake that I would like to get ahead by making by buttercream now.  I have recipe I like, but am not really sure how much to make.  I haven't had enough experience to gauge what I need. 

I am doing a 3 tier cake.  The base is 18x18 square and 3 layers.  The middle tier is a 14 inch round and the top a 10 inch round.  All are going to be 3 layers and covered in fondant.  How much buttercream do you think I should make?  I was thinking about 3 batches, where each batch yields about 9 cups, based on my recipe.  I'm trying to keep the costs down.  So, I don't want to over prepare.

I am hoping to go shopping for supplies today.  So quick responses are appreciated. Thanks.