How many boxes of cake mix for 3 tire cake with 2 layers each, pan sizes are 8x2,10x2,12x2

and 28 cupcakes

also how many tubs of icing  

AI would say around 10-11mixes...you would use 3/4 a mix in your 8, 1 1/2 in your 10 and 2 mixes in your 12...I get about 18 cupcakes per mix, but I like my cupcakes big and fluffy! I am not great with icing amounts, but check out Wilton's wedding cake data chart (just google that) and I believe it gives icings amounts

AI would say 7...

Let's see if I remember right!  8x2 round needs 2/3 to 3/4 of one mix.  10x2 round uses 1 mix; and a 12x2 round takes 1& 1/2 mix. That's what I used for all my cakes when I had my bakery. I found if you use more batter than that in a pan it rises way over the top of the pan, sometimes spilling making a mess of the oven and you have to level the cake, wasting  all that extra batter/cake :(  By baking each layer following the direction I give in my WASC recipe (link:  http://cakecentral.com/t/584319/the-original-wasc-cake-recipe  ) you get a cake that is level w/the top of the pan and a very moist cake that seldom needs leveling.

 Then for the 28 cuppies another box + what is leftover from the 8"ers.

So: you will need 8 or 9 boxes of cake mix to make a tiered cake.

As for how much icing the rule of thumb is one recipe using 2# powdered sugar for each box of mix used.