Now that it came into my mind, I can also give you an idea how to make it via scripting, if you insist. Of course it's total waste of time compared to the way War Master explained. (Of course I have used myself the counter method, but anyway. One can always build nice theories).
First, perhaps in the player spawnscript, your declare a float
variable. Then, in each deathscript of these three unfortunate victims, you add one to the variable (+1). But in addition to that, you also check the current status of the variable. If it has reached some certain value (like 2 in this case, if you start with a zero), you use
a scriptrunner to launch the appropriate script, that should be lauched after all the dudes are down.
For each NPC, in their entity windows, enter key:closetarget value:targetnameoftarget_counter.
I used NPC_target
key, but I guess it makes little difference.