First of all, it is being developed and published by Sony Computer Entertainment and is developed for PlayStation 3 exclusively. You can play God of War 3 on Playstation 3 only and is not going to be released for any other platform just like previously released games of the series which have ben developed for single playstation platform only. God of War 3 release date is not finalized but it is sure that it will be releasing in Fall, 2009. Rumors are God of War 3 release date is set for October 1, 2009 but this is just speculation.
Now let us talk abot the game play and development of the game. It is a hack and slash action adventure game based on the greek mytholigies. The game in fact shows why there were no more Greek mytholigies. The main focus of the game will be to find whether man needs God or not. Game also focus on the war between Gods and Titans which will result in the domination of one over other.
You can play God of War 3 in single player only and in full high definition resolution. It may be that co-operative mode get added to the game, but it is yet not confirmed. The game supports sixaxis tilts and vibrations. It is believed that it also supports Dualshock 3. The tagline used for the game is "In the end, there will be only chaos" and this tagline is well understood from the trailer released in July last year.
Well regarding God of War 3 demo no news is available. Hope there should be playable God of War 3 demo as was for Chains of Olympus. But seriously speaking I don't want to make any speculations for game, so wait and see what all comes under this game in the coming times.By the way we will tell you when God of War 3 full version download or in retail relases.
Update: In an Official Playstation Mag, God of War 3 is said to be released in Fall 2009 which means that game may gets released in October. However actual release information for God of War 3 will arrive at E3 only.