You just need a different macro bound to the same key on each shaman.

ie: shaman 1 has the macro

/cast [target=Toon2] Healing Wave

shaman 2 has the macro

/cast [target=Toon3] Healing Wave

shaman 3 has the macro

/cast [target=Toon1] Healing Wave

Then put these macros in the same spot on each character and 1 keypress will have all 3 shamans healing each other.