I posted this to x360a.org as well. This achievement glitched for me pretty bad. I was able to get it fixed by using the following solution:
OK all here is the deal; this game is glitched...bad. The update that was sent out locks the access to the bunker. You will have two options to get into the bunker:
1. Delete the update and restart your game. Play offline until you access the DLC. Save at any point after you enter the bunker, log back into live and accept the update.
2. Delete the update and reload a save (if you have one) from before the update.
With the update you will not be able to access the DLC. Without the update you will get stuck at the fountain and Elijah will keep yelling at you telling you to get all three people there.
I tried three different restarts without the update. The first one I ended up having Dog follow me for the other two. I could not get him to stop following me because it would not give me the option to talk to him. Tried switching back and forth between his alter egos and it still wouldn't work.
Reloaded the save before I entered the DLC (still no update). Tried all three in the same order - the difference being I sent all three to the fountain without me and did not use their help. Same result - could not activate the fountain.
Reloaded the save before again and picked the people in a different order still sending them to the fountain. Same issue.
My final attempt before giving up I logged into live and accepted the update. I reloaded the save before I freed Dog. Since I had the save I was able to enter the DLC. I freed Dog and sent him to the fountain. Christine was already there, as I passed on the way to the final helper the two of them were having a conversation (interesting since she can't talk). That had not happened on any of the prior attempts. After collecting the third helper - I sent him to the fountain without me just in case. When I got back to the fountain I was able to trigger the next steps with no trouble. To be fair though I have not finished it yet. When I do I'll post if I find any other issues.
If you do have to restart the game make sure you play offline and use the level up glitch at Helios One. Make sure you pick light step somewhere along the line. If you have the opportunity take the rad child one (the one that makes your radiation level decrease on its own - not a lot of rad away in the DLC and your rads go up pretty quick when you eat the food. After you hit level 30 start the DLC and save after you get into it. Quit to the main menu, go to live and download the update. Reload the save where you are in the DLC (I mean in the DLC after you are knocked out - I don't know that it will make a difference if you save after you enter but don't get to that point - I really wouldn't take the chance. When you start playing after you accept the update the level cap does increase (the update fixes that too) so you will start to get xp again and will continue to level up to 35 - higher if you have the other DLC on there as well. I hope that helps others. I checked multiple threads and sites and could not find a good answer.
**Edit** The quest completed after I was able to get all three back to the fountain and progress through the dialogue with Elijah. Didn't pay any attention to it but the achievement never popped. I finished out the DLC and everything else worked as it was supposed to and the game still showed that I had not gotten the achievement. I deleted the DLC, cleared the system memory again, deleted the update and reloaded the save before the bunker. After I got into the bunker I logged back into live and continued the quest. I ran through pretty quick (having done it a few times I knew where to go). Dog first - sent to fountain, Christine second - sent to fountain, Dean last - as soon as I told him to head to the fountain the achievement popped. I like this game but I am pretty glad I am done. I also used the video and was able to escape with all 37 bars of gold. Really no need for them since I am done now. I didn't actually go back to the wasteland after getting the final achievements though. Again, I hope this helps others and this game is going on the shelf.
