Wow, Sony actually has a good idea for a change. Just wish I could cleanse my mind of the foulness that was Killzone 3's crap SP. A shame KZ3's MP was nowhere near as good as KZ2's too. Still, hope to see more of this (I'm looking at you EA).
Does it already unlock if you own Killzone 3? probably not.
The biggest problem with Killzone 3's multiplayer (aside from the horrid map design) is that everything is too fast. Character movement, fire rate on all weapons, everything. If they slowed everything down akin to the speed of Killzone 2 but largely left everything else the same, it would be a much better game.
You prefer KZ3 multi over KZ2? To me it felt like they stripped away the heavy feel from 2 that made it so unique. Now it feels like any other shooter. They also condensed some of the classes and changed the way things unlocked and I didn't particularly care for it. I haven't played after the last few patches though so I'll have to jump in and see how it plays.
Multiplayer-focused games like this should sell the single player mode at retail for cheap and sell the multiplayer as DLC separately. More money for publishers and developers, less for Gamestop, and it's not like anyone without an internet connection is going to use the multiplayer anyway.