Hi Oliveoyle,
You will have to bake the correct sized cakes to feed 300 and it may not be 3 tiers, you may need 5 tiers, depending on your sizing. I would use my home & hobby pricing matrix, or Alice's professional matrix to determine how much ingredients will cost you and what size pans will feed how many people, etc...
Attached is the file. You will need Microsoft Excel 97' or newer to open. Good luck!
PS-As to design, a simple cake can have some pretty white bordering. If you are new to cakes, check out Wilton's wedding cake book or Martha Stewart wedding book which is new. The MS book has some great, simple cakes.
thanks for the advise, not new to cakes just haven't done many wedding cakes.not sure I made myself clear she wants three separate 3 tier cakes to feed three hundred and I'm not sure what size to make the tiers will check out the matrix as you suggested
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