the couple wants 3 tier round, what size pans should i use?

Well, that really depends on several factors.  What size cake slices would the couple like to serve?  Wedding cake portions are generally smaller than dessert size portions.  Does the couple want to save their top tier, or do they want to cut and serve it?  Will there be other desserts served at the wedding?  Will they serve a full meal or a buffet?  What time of day will the reception be held?  How many flavors will the cake be?

All of Patrinicia’s questions are good ones and the same I would ask.

In the Cake Bible, Rose recommends an 18x12” sheet pan for 80 people, but if your couple wants a cake with tiers, I think two round tiers (the bottom using a 11 or 12” cake pan and the top using a 7 or 8” pan) would be pretty and plenty of cake. For three tiers, you might want to do a 4” top tier, 7” middle and 10” bottom. According to Rose’s charts, you would be left with a extra cake.

I did a 10” bottom tier and a 6” top tier for a wedding of 50 people, which was supposed to serve 75, so I thought I was golden.
But the people cutting weren’t caterers (who are used to making small “wedding~size” slices) AND they were hungry because they were only served appetizers so people got hefty slices of cake (which was already a heavy chocolate torte with raspberry buttercream and chocolate ganache!) and the cake was GONE.

I don’t mean to make anyone else planning to bake a wedding cake worry, though…..If the wedding is catered, the caterer will slice the cake correctly and make sure there is plenty. It’s not typical to run out of cake.

Personally, I find TCB portions are too small for my area.  I portion my slices to be 1"d x 2"w x 4"t, and have made my own serving chart and rectangular cutting guide (even for round cakes) - According to my chart (1x2x4” slices), a 6-8-10” cake would be enough to serve 80 exactly.  So would an 8-12” cake.  Since the couple wants 3 tiers, you could add a fake bottom layer from a Styrofoam dummy, decorated to match the other tiers…. just some ideas to throw out at you.