Four new maps are being added to Killzone 3 next week via the next DLC pack, "From the Ashes." Players will get two Guerrilla Warfare maps and two Warzone maps. The third downloadable add-on to Killzone 3 will be available on the PlayStation Store on June 21st for $4.99.
If you haven't purchased any of the DLC so far, you might be interested in a new bundle that will also be available on June 21st. The DLC Bundle Pack combines "From the Ashes" with the two previously-released map packs--"Retro" and "Steel Rain"--into one package that costs $9.99. It's a rather reasonable price for eight new maps.
(Note: Current PlayStation Plus members [i.e. almost everyone] can also download "Steel Rain" for free right now. The deal expires on June 20th.)