I have a cake to do this weekend and I am using the Fat Daddio topy turvy pans.  What is the best way to stack them?  Do I need to hollow out the tired beneath?  It is going to be a 3 ter cake.

Yes!  hollow out a space that is level and the diameter of the base of the tier above.  Each tier should be sitting on a level separator with supports. 

I own these same pans & I use them pretty often. I never hollow out the tier beneath. I stack them like I do any other cake... put the cake on a board, ice the cake, add dowel rods (cut to height of cake), stack cake, repeat then when it's all stacked I add a center dowel through all 3 cakes for transporting. If you were making/carving the topsy turvy yourself & not using these pans then you probably would have to, but not with these pans. You can if you feel better about it, but you don't have to. I have done several 3 tiered cakes with them & have never had to hollow them out.