Maybe if yellow was adjacent to the battling regions?
What would be in it for the yellow player? A war chest?
Could be, maybe one of the two players could pay the 3rd player off. The 3rd player couldn't donate any cubes to throw into the tower but could say that any that fall out would count toward who they chose. The risk would be that if the team they helped didn't win they would lose theirs also. I like this idea just have to strategize it out more.