First in order to unlock the 3rd Riddler Hostage Mission You will need to find 160 of the Riddler Trophies. Thanks to LucidDreams84 for the Heads Up on that.
Next The Enigma Machine will pop up and the answer to this is "Penny" Rotate the gears so that you make the word penny and then you will get the location of the Third Hostage.
Once you get to the Building, first you will have to make your way across the frozen water The easiest way to do so is the Line Launcher though you can also use Freeze Blast. Once there you will see two Question Marks. above them are some Generators. Use an electrical charge to close the gate over top of the Question Marks one at a time stopping the waterfall in front of them then use a Remote Batarang to hit one and a simple Batarang for the other. Now Get out your line Launcher and Head to the Right. Use the Launcher to zip your way through the tunnel but halfway through you will need to switch directions to your right once again. Once through that... this next part is easy enough. There are Three Generators. Fire a Pull charge to the one on the right then immediately fire another Pull Charge to the Generator on the Left. Then Fire a "Push" charge at the same generator and then another "Push" charge to the one in the middle which should push the crate over the top of Pressure Plate... and there you have it. Hostage Rescued and 30G
On my next Play through of the game.. I will try to make a video. Gotta finish New Game + first