Similar to Horizon, Modio was a staple of the 360 era for accessing and "re-hashing" save files so the console recognizes them after they've been edited.
There are several standalone .exe editors specifically designed for Fallout 3 that allow for more granular control over quest flags and NPC relationships. How to Edit Your Fallout 3 Save: A Step-by-Step Guide Step 1: Preparation fallout 3 xbox 360 save editor