try
Trigger (any width, just the default options are fine)
Name your scud units, eg scud1, scud2, scud3.
In the activation field, where it says 'this', delete the 'this' and put ' {(scudState _x) > 3} count [scud1, scud2, scud3] > 0 '. Above this field in the trigger, set the delay type as countdown, and the delay max, min, and mid to whatever time period you want the mission to end in once a scud has been launched
Hope it helps