A spokesperson for Bethesda has told Eurogamer "there are no announced plans for additional DLC beyond Broken Steel" for Fallout 3.
The publisher was speaking after US retailer eStarland outed a fourth add-on called Point Look as part of a DLC bundle with Broken Steel. A similar retail deal containing Operation: Anchorage and The Pitt was confirmed to Eurogamer earlier this week. There's no further information about Point Look on eStarland's website.
Another claim supporting this is that Bethesda has only used 350 gamerpoints out of a possible 750 so far, and only 14 Achievements out of a possible 30 as established by the new DLC rules confirmed this morning. That suggests Bethesda has left plenty of room to grow Fallout 3 further.
Broken Steel launches on 5th May, costs 800 Microsoft Points (GBP 6.80 / EUR 9.60), and raises the in-game level cap to 30, most notably.
There's much more besides, of course, that our Fallout 3: Broken Steel preview will fill you in on.