Let me Gasp you up.
FM9 on it's own is great. Love it. But it's big. And if you want to add anything else to it; it becomes cumbersome, imo. The FM3 for me is a Stomp replacement. Post fx and routing that will sit on top of my amp with the GM on a small board. I can bring the FC12 with me or just the FC6, connect my expression pedal and have a couple of different available footprints to control it all with, depending on how big the "stage" is.
The 9 and the FM3 seem to be on the same update cycle. So it serves a completely different purpose from the III which will stay at home and be the center of the cr@ckhub.