I wouldn't ice it in buttercream because the three milks cake is way to delicate for a heavy frosting. I would use a modified whip cream or I've used the whipped cream buttercream recipe from this site.
You aren't stacking these on each other are you?
You will have to use a system that doesn't have the cakes stacked on each other. I would use a cascade type of support for the cakes or the "floating" tier system from wilton for the three milks cake.
Check at a local bakery to see if they get any of their materials from bakerycraft and see if they would order you either the five tier or 3 tier cascade delivery system. Or check out Pastry whiz and see if they have one ( I think they do). Here is the URL for pastry whiz www.pastrywhiz.com
I wouldn't use buttercream, here in Guatemala we eat the cake with "turron" it's delicious !!!! i'll try to get u the recipe, before tomorrow but if I can't search for turron(spanish), it is also very popular for fondant cakes, almost every wedding here is asked to be done in cajeta or turron ... u should try it !
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