How to set a special creep spawn after specific creep death...

I wanted to do this for my map.

I have three bosses on my map see, and I wanted(well my bro wanted but I thought I should give it a shot) to do this:

After the three bosses die(as in all of them, not one, not two, but three, yes the three of them), a final boss would appear. All i've managed to do is, when one of them die, the boss appears. But after I kill the three, I end up having three final bosses. I've been working on it for days to no avail of the correct procedure.

Can anyone help?

make a global array of booleans:
bosses_dead

each boss has an index in the array, like the first boss is bosses_dead[0], ext.

When a boss dies, set its boolean to true. Check to see if all the booleans are true after that, and if they are, spawn the final boss.