Wilton has a great serving chart on their website. To feed that many and have a 3 tier you'll need something like 8/6/4.
The Wilton chart is great, I laminated a copy of and refer to it for everything from cups of batter needed to number of servings.
That's a great idea. Everyone should have one of those posted in their kitchen 
Someone from here on CC had came up with this cool tool that uses the Wilton serving chart and posted it here on CC for all to use she called it calculate cake:
I have it Bookmarked in my toolbar this is a life saver for me cause it does the calculating for you. and since you hate figuring it out this will help a lot.
http://shinymetalobjects.net/cake/calculator/cake_calculator.cgi
6" and a 8" is already 32 party serving size add a small 4" which is about 2 servings you'll have 34 serving.
hope this helps you.
Shaddi
Shaddi, I knew that those sizes would produce a few more servings than the op needed but it's pretty close and most people have those size pans
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