Is there a trick to stacking three or more layers, (not tiers) because I have tried two cakes, one with three layers and the other with four and both of them sagged more and more each hour. They looked great after I covered in buttercream, I chilled them in the frige for several hours, but after the fondant went they they began sagging and bulging. I put dowels in one thinking that might help, but it didn't. Any advice?

What recipe did you use? How high were your layers? How much buttercream and what buttercream recipe did you use? Did the problem start in the fridge or how warm/hot was the room?

Did you take photos by chance? If not, try to remember where the bulging happened - in cake or in the buttercream filling.

All my cakes have four layers with buttercream in between and I don't have this problem, even through I use sponge cake. Your cake should be way, way firmer...

Did the cake start to sag still chilled or out of the fridge?
You should get the same problem if you use 2 or 4 layers, it works the same.

I can imagine a buttercream problem - it is too warm for buttercream, it starts to melt and the weight of the fondant and top layer of the cake press it out of the cake. You would have the same problem with buttercream decorations - for example roses would get dropped leaves.

Dowels won't help - they should support the next tier, not the layers themselves. They help to stop the layers from sliding to the side, through.

For melting buttercream, there is one help only - keep the cake chilled, either in the fridge or turn the AC up. Maybe your buttercream recipe will never work and you have to find another one.

Rarely, the problem is with the cake - you would see this as well. The buttercream would stay where it should but the cake layers would be deformed. This can happen if you have high layers from very airy cake (I hope you understand what I mean). This can happen with sponge cake - it works than as a real sponge - it can compress in the middle and bulge to the sides. The only real help - other than making cake from different recipe - is to stack the cake back in the cake pan, (lay parchment paper or a plastic wrap in it first) lay a cutting board on top of this and throw some weight on it (juice in a box or something similar works nice). Let it stay overnight in the fridge. The cake compress in the form as much as it can and after you take it out, you can crumbcoat it and decorate as usual.