I have not come across serving guide for 3inch deep pans. Not sure if they are the same as a 2inch deep pan, which I have already looked??? Also, if you stack 2 cakes the same size to equal 1 tier...how would you determine how many it feeds??? Hope this makes sense...Any information would be helpful 
im pretty sure wilton have that on their site, sorry i dont have the link saved on this computer.
xx
as far as stacking 2 cakes for one tier, i think the general consensus is that you get the same number of servings, just twice the cake height,.... since to make a real impact on the # of servings (say, to double them) you'd have to cut each slice so thinly that it would be counterproductive...
wilton's 3 inch deep pan serving guide has the same numbers as the 2 inch deep pan...however, it states this:
"Serving amounts are based on party-sized portions of approximately 1.5 x 2 in. Cakes from 3 to 6 in. high, baked in the same size pan, would yield the same number of servings because they follow the same pattern of cutting. Cakes shorter than 3 in. would yield half the number of servings indicated for that pan. Number of servings are intended as a guide only."
it says the same thing in the 'wedding serving guide' also
i keep this link VERY handy!! i am ALWAYS referring to it http://www.wilton.com/cakes/making-cakes/
on the right, the 4 serving charts are under "baking time and batter amounts"
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