The biggest problem with mouse input is that it brings the window to the front causing a performance hit and blitter.

When I'm three boxing how I lay down blizzard is by simply moving to each screen and clicking... It staggers them a slight bit, but I've got it down fairly well.

One thing I found was that having the view set the same was insanely useful... So I made a macro:

/script SetView(2);SetView(2);
/cast Flamestrike

Why SetView() twice? If you play with the keybindings for views, you'll see that pressing it once slides you into place, this slide is very slow for combat even though it's slick looking... press the key twice quickly and snap, there's your view. Sending the same command twice in a script accomplishes the snap move instead of the slide move.

So I have G11 set to send my mages on a flamestrike mission... I hit G11, mouse to WoW2, click twice (Once to set focus, once to cast), repeat with WoW3, and back to wow1 for playing the paladin. This would be exponentially easier with mouse broadcasting, but this is certainly feasable....

One thing that helps with this is the fact that my AE, even targeted, generally need laid down right at my feet. The reason for this is I'm using a pally tank who's got amazing threat generation...