My favorite cake size for 3 tier is 6, 9, 12. This should feed about 70 then I would do a sheet cake for extra servings.
Just my 2 cents...
by the way do a search for "serving amounts" you will get a load of info on what to expect for each cake size.
Here's a link to Wilton's serving guide:
http://www.wilton.com/wedding/cakeinfo/cakedata.cfm
Here's a link to options to serve 100 to 150:
http://www.wilton.com/wedding/cakeinfo/choosing/100_150.cfm
I also go by the wilton wedding guide. I also never do sheet cakes at a wedding. It's just bluntly never really come up, once the bride finds out she can get a big, grand wedding cake for the same price as a sheet cake.
I also prefer making a big, grand wedding cake! The bigger the better!
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