I seem to remember a post where someone was doing /targetenemy and using /targetenemy 1 /targetenemy -1.

Not sure if the -1 option works (or if it's even that particular command)

Basically you could hit a key that targetsenemy on 1 toon, does targetenemy 1 on another and if it works /targetenemy -1 on the third. Perhaps even setting that enemy as the toons focus.

Your main could target a 4th manually and focus and then you could mass CC. Probably wouldn't work as smoothly as that.