someone wants a 3 tier cake for 30 people. I dont think it possible. I have not idea what pan sizes to use....any suggestions?

A 4",6",8" would get you close. A 6 and 8 is 36 servings, plus I'm guessing 6-8 more for a 4"? But I'd still be charging them for the full number of servings.

you can get WELL over the amount of servings of 30 people for a 3 tiered cake! just a 6" and 8" round alone will serve 34. the next size would either be a 4" for the top or a 10" for the bottom...and you definitely don't need a 10" cake.

sometimes you have to educate the person receiving the cake on how much cake they need (if they are ordering)...they may THINK they need 3 tiers when in fact they could only do one 2 layered cake, 8" square.

Can one tier be a 6" styrofoam circle or a cake dummy? The other tiers: top a 4", bottom tier a 8". Just a thought!

If a customer wants the look of a three tiered cake then you work with that. Tellng them "they only need 2 tiers for the servings" is totally missing the point.

You could possibly do like someone mentioned, educate the person on the servings that each tier renders and if its the height they are looking for , I did this cake for a babyshower of 30-35 people and this is a 3 -full 2inch layer 8 inch round with a single 6 layer on top

http://media.cakecentral.com/gallery/45193/600-1318549805.jpg

Yep, 4-6-8 x 4" is exactly 30 servings (at 12 cubic inches per serving)

First tell them how much each size pan will serve. Then tell them about what size for each pan you would need to get their total number. YOu could always offer a dummy cake for the bottom or top tier, charge them your dummy price, then get your total.
I had a customer tell me she wanted a cake to serve around 25 for this fourth of july. I emailed her the picture of the cake (replica) and she didn't think it would feed 25. She said "well, our family loves cake so I just want to be sure there is enough." I emailed her the actual Wilton cake pan serving chart and told her if she wanted to be sure there is "enough" cake she would have to order more cake, i.e. a bigger cake with more servings. Some people don't understand that a serving size is the standard and you can't account for huge servings per person just because they like cake.