I am making a cake that will be just 3 12-inch layers stacked on top of each other. It will have a figure made of RK treats on top of that. Will the weight of the cakes themselves cause them to flatten or fall in on themselves? If so, what should I do to prevent this? I have the picture of the cake in my head and I don't want it to end up a mess in the end.
Thanks in advance for any advice!

My rule is anything taller than 4" gets support. Though I have been known to go 5" on occasion. I wouldn't go any more than that.

In that case, do I need to go ahead and make it 4 layers so I can put support between them? With 2 layers beneath the support and 2 layers on top of the support?

or you could torte your 3 layers so you have 6 tortes. Then you could have three tortes below and three above the support.

Not at all !!! I did one recently the same as you and as a matter of fact using a 10 inch cake board I carved down for that tapered look flipped it and it was perfectly fine !!!

Artscallion, you are so wise! I thought of that and wondered if that was an option but then just assumed it wasn't. Good idea!

Are you talking about layers or tiers?

Just layers, no tiers