Do you need say M3 for M3 stuff to fit or just any of the gen3?
Also how long will they be on sale for?
The iconic shapes are to make Genesis 'look like' M3. When you buy the shapes, you also get a clone for the Autofit tool so that you can use existing M3 clothing on the changed Genesis figure as well. Genesis clothing should already fit any of the iconic shapes.
My understanding is that you need the M3 shape for Genesis if you want to fit M3 clothing to Genesis. You don't need the original M3 figure (but you can get him, V3, A3, and H3 for free at download.com, last I checked). If you only get the A3 and H3 shapes, though, you won't be able to AutoFit M3 clothing to Genesis...just A3 and H3 clothing.
Info about the sale can be found here: http://www.daz3d.com/forums/discussion/5495/