Last Man Out.
You'd have to chain all 3 I suspect, so each one spawns after the previous, which would force the last one to literally be last... Sadly at the moment most of the really fun things we'd like to do isn't possible in the Mission Architect, so we have to find work arounds; in your case, to keep the mission fun, a fixed map, and using Front, Middle, Back spawning so the player doesn't have to go back and forth across the map looking for new hostages would be desirable...
Let's say that there are four Escort or Captive objectives. One of the NPCs is the leader, and refuses to go until the others are all safe. How can I make it so that one Escort/Captive cannot be rescued until the three others are?
Here's a similar issue: a Escort/Captive is a hostage who won't be brought into public (that is, the objective won't be triggered) until the heroes finish three other clickies/destructibles/defendables. They can do them in any order, but all three must be done. Any ideas how I would do that?