According to a post on the official PlayStation Blog, Killzone 3 will welcome the Steel Rain multiplayer map pack beginning April 12. The content will sell for $5, but PlayStation Plus subscribers can download it for free until May 10.
As for the maps within the content, Steel Rain is composed of two new environments based on Killzone 3's single-player campaign. Junkyard is a guerrilla warfare map that has players duking it out in a drenched garbage processing factory. The other map, Stahl Arms, is a warzone map set on one of "Helghan's largest weapons manufacturing plants."