I have a three tiered cake to put together  the middle tier is actually sports balls that will hold the top tier  anyone have any suggestions on how to support the top tier? what the sports balls are made out of? And how to keep them from sinking in the bottom layer? 

If I understand your ?s right I would assume those balls are styrofoam   Dowels would be put down through it all the way into the bottom board   Of course the top tier is on a cake board so the dowels hold it up   

Ditto what kakeladi said!