Fallout 3 is Bethesda's post apocalyptic RPG game which has been released for PC, Xbox 360 and PlayStation 3. The game is set 36 years after the events of its predecessor Fallout 2 and after 200 years of the nuclear war in 2277. Player is given the role of safegaurding small group of humans from the nuclear fallout. So game follows the storyline similar to its predecessor.
It is to be noted that there hasn't been any Fallout 2 demo as well, so how there could be a Fallout 3 demo. Fallout 3 demo i not available for the either version of the game viz. PC, XBox 360 and PS3. However there is lot of free Fallout 3 downloadable content and premium DLC available. The GECK, Operation Anchorage, The Pitt and Broken Steel are among the downloadable content available for the game. More downloadable content will follow soon.
Why it is that no Fallout 3 demo has been released? If we go by the words of Erogamer, Pete Hines of Bethesda has confirmed no demo for the game. According to him, "When you build it as one thing, there's no way to portion off a section and have it stand on its own without putting the whole game in the demo, which we're just not going to do,", so you may have understood why there is no Fallout 3 demo for PC, XBox 360 and for PS3 even.
Fallout 3 was code named Van Buren when it was under development by Black Isle Studios and a playable Fallout 3 demo was released by the developers that destructoid has mentioned about on their website. However, this tech demo was of the cancelled project. So there has been no Fallout 3 demo for PC, XBox and Playstation 3 in actual. And there will never be any Fallout 3 demo for PC, XBox 360 and PS3 ever. So stop dreaming about Fallout 3 PC demo and other platforms and buy its full version instead. No Fallout 3 demo ever!