Maybe you mean that the graphics will be 3d models which are rendered to an image (png or the like) and used as sprites, instead of making 2D sprites with pixels or vectors?
Anyway, I'm guessing what you want to do is have an object fall from the celing when the player walks under it?
You could do this in a number of different ways depending on how you control your game. But what you want to do is check if your players X position is equal to, or close to, the object that will fall, and when he is you have another piece of code actually making the objects fall.
I made a small cap with a very basic implementation of this.http://www.box.net/shared/0pcbmfipzc
Also, for future reference, threads about getting help with a problem should go in the "Help / Tech Support" section of the forum.