I would do it the same as a regular topsy turvy cake ~ maybe carve the bottom of the base cake to make it more narrow on the bottom than on the top. There are several pictures in the gallery of TT cakes using only 2 tiers:
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=631263
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=628193
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=594873
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=590283
These are just the first few that I came across. Take a look in the Whimsey/Topsy-Turvy area of the gallery for more ideas. Good luck!
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