I have a cake comig up and the tiers need to be triple layer to get the height I need for the design. I have never done one this way, and need some advice on the construction of the cake, do I need to anything differently? Also how do I figure out how many servings are in each cake? With the slices 6 inches tall instead of 4, I assume they should be cut thinner than one inch wide. Does anyone have any experience with this????

That depends. Nothing differently, if you are serving 6" tall slices. And, it would serve the same (according to Wilton) because it would follow the same pattern of cutting for cakes from 3 to 6 in. high. (nearly impossible to cut thinner than one inch.) You should consider that 6" tall cake slices won't fit on dessert-sized plates.

If you are considering serving 3" tall slices (with a cake board in the middle at 3"), then the construction is slightly different... 3" tall pegs, board, 3" tall pegs. And, it would serve double the amount, just 1x2x3 inch (6 cu. in.) servings instead of 1x2x4 inch (8 cu. in.) servings. Plus it fits on the plates! HTH.