Won't I need to /setfocus or something like that first?

I wasn't too clear I think.

I want to press button 1 for shaman 1 to set that as their focus, press button 2 for shaman 2 to set that as their focus, press button 3 for shaman 3 to set that as their focus, then press button 4 for all three shaman to cast Hex at their focus target, then of course assist me as we kill other things, then when hex cooldown is done I want to recast all hexes by pressing button 4 again, making them /stopcasting and cast hex on their focus.

So your
/Stopcasting
/cast [@Focus,Exists,Harm] Hex
will work perfect for that last part, but I'm not sure how to set up a RR in ISboxer, and thought it would be easier just to have seperate buttons for each shaman. I just got a G110 keyboard so extra keys are not a problem.