Sony announced today that Killzone 3's multiplayer will be available as a free standalone download on PSN. You will be able to download the multiplayer sometime soon. So just to confirm, this doesn't include the singleplayer campaign as they plan to sell that separately. You'll be able to download Killzone 3's multiplayer next week.
The download does feature a level cap, but players have the option of paying $14.99 to unlock the full skill tree once they reach that maximum level. It also features all of the standard Killzone 3 multiplayer maps.