hm, from a first glance your setup seems ok for me and should work!
So, the only thing which might cause something like this, i can think off, would be, if you have bound 'shift/alt/ctrl + i' to something other, so that the macros don't activate.
But even then, normaly the two slave toons, shouldn't get the 'no target' message...

..could you perhaps try to change your macro, so that it doesn't assists but casts directly on the target and see what happens, like:

Code:
/cast[mod:ctrl,mod:shift,nomod:alt,target=Blastonce-target][mod:ctrl,mod:alt,nomod:shift,target=Blasttwice-target][mod:alt,mod:shift,nomod:ctrl,target=Blastthrice-target] Fireball;
(im not exactly sure if it is 'Blastonce-target' or 'Blastoncetarget' so you may have to trythat out)