I suppose my problem could be caused by an ATI driver issue ...
I'm now pretty certain my problem is not caused by MA3, but is in fact an ATI driver issue. It occurred to me to try another screensaver (Microsoft's own 3D Text) while I had two monitors connected to the HD3450 card, and I got the same (unhappy) result. So, I guess MA3 is off the hook. (Sorry for wasting your bandwidth.)