This is part of the Fallen Angel DLC.This seems to be the third most daunting achievement in Dead Rising 3, with a whopping 1.1% attainment rate. Now, there are eight total survivor groups and in order for a rescue to count, you must see the Mission Accomplished indication for it to count. If someone dies while they are untied or dies before they are untied, it doesn't matter so long as one of the three in a group survives. Below is a map showing all of the locations. The survivors only begin to spawn once you get close to the first group, so stock up before you head out.
Refer to the below image for specific locations:
http://images.akamai.steamusercontent.com/ugc/57569222546202...