You want the bottom tier of a three tier to be smaller in diameter than the tier about it? What size is the top? I'm not quite grasping what you're describing.
You can make a topsy turvy where each tier is made up of the layers of the same size and then cut the top layer at an angle to make the TT bit. Or you can make it so that each tier is made up of three layers of decreasing size (ie. 10, 9, 
where you would carve the tier sllightly to even it out. If you do the latter method, then you could make the tiers as follows: bottom (10/9/ middle (8/7.6) top (6/5/4) where the bottom layer of each tier is the same size as the top layer of the next tier (if that makes sense).
If you look at my photos, you'll see the topsy turvy I did like that.
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