The latest PlayStation Blogcast has confirmed the multiplayer suite of Killzone 3 will arrive "standalone" on the PlayStation Store next week for free with a level cap.
Once you reach the cap, you'll be offered the option to lift the cap and continue playing for $15. It'll feature all of the maps included as standard.
To reiterate, it's multiplayer mode only. The single-player isn't included in this.
This is only for US right now, but we'd likely think this is for the European update next week as well.