The game picks up immediately after the ending of Killzone 2 and follows the previous games' main protagonist, Sev, a Special Forces operative fighting for the Interplanetary Strategic Alliance against the Helghast Empire. The game's story has been referred to being similar to the story of David and Goliath. The game continues shortly after Sev successfully defeated Radec and Rico having killed Visari, the Helghast dictator who leaves them to a fleet of Helghast ships approaching them destroying ISA vessels.
The death of their dictator has left the Helghast Empire in a state of internal struggles with political scheming against each other gripping the Helghast Empire. Now, Sev and his comrade Rico are trapped in the cross-fire between the various political factions in the Helghast Empire without any reinforcements and are left to fight alone in order to escape as the ISA struggles to regroup its forces. The game will also give players a deeper look in to Helghan culture such as their language and showcase whatever humanity they have left within them.
Killzone 3's story will also be a lot more light-hearted as compared to its predecessor which Guerrilla felt suffered from an overly serious and dark story which was one of the major complaints of the original, though they mentioned that it would not be too light-hearted as it is still a war. In addition, Guerrilla Games has hired an external writer to focus on only writing the storyline so as to give a fresh perspective to the story as well as hiring a lot more Hollywood actors to elevate the level of storytelling within the dialogue of the game.
Guerrilla Games has also mentioned that the game's story will be an integral part of the game with dialogue and character development being greatly improved from its 2009 predecessor. In addition, Guerrilla Games has also said that they are curtailing the swearing in Killzone 3, which many felt were unnecessary in the dialogue of Killzone 2. They mentioned that the dialogue in the game should be focused on advancing the story rather than "gratuitous" off the scale swearing without reason.
The gameplay will strongly resemble that of Killzone 2 albeit with several tweaks and changes. Unlike previous games in the series, the game will take place in various harsh environments such as Arctic landscapes, a lethal alien jungle, nuclear wastelands as well as space battles rather than only the city-scape environment which were present in its predecessors. Another notable new feature is the ability to use jet packs in the game, which were seen in Killzone: Liberation, however, these new jet packs will be used in a different manner than that of those that appeared in Killzone: Liberation.
The game will also feature a revamped close combat melee system which include new brutal melee attacks as well as the ability to string together multiple attacks in a combo. Unlike its predecessor, Killzone 3 will feature fully destructible environments which is an improvement from its predecessor which featured only partially destructible environments in limited areas. A new weapon called the WASP has also been revealed, which acts as a multi-firing Rocket Launcher. In the July 2010 edition of GameInformer Magazine, it was revealed that the WASP rocket launcher will have a primary fire and a secondary fire, which leaves to speculation that secondary fire for various weapons will return as seen in the original Killzone. thumb|300px|rightthumb|300px|right
The game was officially unveiled to the public by Sony Computer Entertainment on May 24, 2010 through the PlayStation.Blog. The game was first leaked to the public on May 21, 2010 through an early release copy of the June issue of gaming magazine Gamepro alongside a leaked screenshot. Rumors about a sequel to the Killzone series have been around since just shortly after the release of Killzone 2. It was not until March 20, 2010 that Sony officially confirmed that they were working on Killzone 3. President of Sony Computer Entertainment America, Jack Tretton, confirmed its development in an interview with GameTrailers, but at that point of time could not yet say when the game would be officially announced.
Guerilla's managing director, Hermen Hulst, has stated Killzone 3 will use close to 100% of the PS3's power. The rumors of the intervention of Naughty Dog in the development were denied by Guerrilla: "Guerrilla is working on Killzone 3's graphics engine all on its own, confirming Naughty Dog has no hand in its development" The first teaser trailer for the game was released on June 3, 2010 and features the new brutal melee combat system. The cinematic trailer shows Sev battling a Helghast ending with him taking the Helghast's helmet and putting it on himself.
On June 11, 2010 on Killzone.com and the US PlayStation Blog, Guerilla Games released a pre-E3 gameplay video. Guerilla said the gameplay video is made of gameplay footage pulled straight from the game's fourth mission called "Frozen Shores." It featured the new brutal melee system, the jet packs, the new weapon (the WASP missile launcher), and it featured Sev, as well as Rico, with Sev's hair a bit longer than in Killzone 2. At E3 2010, it was confirmed that Killzone 3 will support the PlayStation Move and will be released in February 2011. A pre-alpha code gameplay video was also shown at E3 in 3D; a special screen was lowered and the audience had 3D glasses available to them. The E3 gameplay video featured the jet packs as well as the player riding on a craft using a turret.
Killzone 3 will include a mode that will support Sony's plunge into 3D gaming. The Killzone 3 3D mode will be optional, and will require a 3D ready TV with 3D glasses. The first impressions of the 3D show some glitches; with three-dimensional visual effects, the game becomes blurry, imprecise, and disorienting. In alpha version, 3D sacrifices things like sharpness, speed, a feeling of orientation and a lot more aliasing. Jagged edges as well as some ghosting of interface elements are reported to occur. At E3 2010, it was revealed that Killzone 3 was built with 3D support from the ground up. In praise of the games 3D capabilities GameTrailers awarded the title Best 3D Graphics of E3 2010, although officially it did not have any nomination in E3 2010. The E3 2010 trailer resolution is half the horizontal resolution of the 2D game, with the whole image (including HUD) scaled up using the PS3 hardware scaler and renderize the vision of a single eye.