AI have to make a wedding cake for my friend on the 21st. But I have to finish the cake before the 20th because I have important business to do. I want to know how I can store the finished cake for about 3 days before the wedding.

three days is kinda long to me to store a finished wedding cake

i have only used marshmallow fondant a few times

i would not finish it that far in advance is my only answer

it depends on the decor too

lotsa pesky variables here

not saying it can't be done successfully i'm just saying i would find a different work around myself

best of the best to you!!!

Like she said, it's all going to depend on your recipes. 

I personally have frozen cakes completed with marshmallow fondant (dark blue w/black) without any problems--but I am a very lucky person. If you live in a humid climate, you could have bleeding--not so much if it's all white. Add gumpaste flowers on wedding day.

I would not have refrig/freezer space for a large cake--make sure to measure--don't forget height AND cake board.