okay. objectives are controlled using the
objstatus command.
as you can see the objstatus can be "hidden", "active", "failed" or "done".
set up a trigger in the mission, doesn't need to have any size, but in the 'condition' box type
not (alive officer1) and not (alive officer2) and not (alive officer3)
and in the 'on activation' box type
"n" objstatus "done"
where
n is the objective number from your briefing.
now when all three officers are dead, the related objective in the briefing will have a green tick next to it.