I recently made a cake in which I stacked three 2" torted layers with just one foam core base under the bottom layer. They weren't very large, only 6" square, so it worked fine. It just got me thinking.....how large can you go without dowling and supporting the 3rd layer with another cake board. I'm guessing at some point the size would make it too heavy to stack 3 layers without having the cakes sink. How large do you dare go?
Ideally one should add supoort (dowels) when ever you go over 4" tall.
Having said that I would go 6" as long as the cake isn't big & heavy. Maybe not over a 10"er.
I always dowel (or straw) every tier.
If it is heavy, I glue together 2 foam core boards.
If it is topsy-turvy or otherwise an awkward stack, I glue together 3 boards and hammer one or two dowels all the way through the entire cake down into the foam core board, in addition to normally supporting the tiers.