From a logical point of view, you should first present the set with the cylinders that vary in three dimensions. However, we do not do so for psychological reasons. If you were to give a child the set that varies in three dimensions, you will find that he can put a cylinder into the wrong cavity and subsequently will be unable to get it out again. This manipulative difficulty leads to tipping the block upside down or hitting it, etc., to remove the cylinder. Such activity would cause the constructive interest to be lost.