If Killzone 3 were to launch in late 2010, it would be an approximate 18 month turn-around for Guerilla, certainly not out of the realms of possibility with the engine already complete.

To quote: While reading a copy of Playstation: The Official Magazine (December 2009) issue that was sent to my mail, something really seemed to catch my eye. On page 20 of the magazine there is an article titled "The Secret Games of 2010″ and it just so happens to speak of Killzone 3.

"Insider slips reveal three big names you can already pencil for the next year. Killzone 3 is reportedly deep in development, Insomniac’s Full Moon Show podcast hints at a project we can only conclude is Resistance 3, and a Treyarch employee’s online mentions a Call of Duty: World
Rumor: Killzone 3 in Development for PS3, Scheduled for 2010

Rumor: Killzone 3 in Development for PS3, Scheduled for 2010 is a post from: PS3 Lite and Slim News

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1. Rumor: Killzone 3, Resistance 3, World at War 2 Coming in 2010
2. Killzone 3 Due 2010?
3. Rumor: Killzone 3 Gets 2011 PS3 Target Date by Guerrilla Games