since they have the engine done, i think its very possible.

I honestly don't think we will see another Killzone in PS3. My thoughts about KZ3 is that it will maybe launch with PS4.

But hey, that's my opinion.

I'm predicting that Killzone 3 will come out Fall 2011 and will be one of the last first party megahits on the PS3 as Sony's first party devs start shifting their focus to the PS4. 

I also think Resistance 3 will come out Fall 2010 to face off (and get smashed again) with Gears of War 3 and the annual CoD game. 

See the trend?

Fall 2008 - Resistance 2 vs. Gears of War 2 vs. CoD: World at War

2009 - Killzone 2 vs. Halo: ODST vs. Modern Warfare 2

Fall 2010 - Resistance 3 vs. Gears of War 3 vs. next Treyarch CoD game (World at War 2?)

Fall 2011 - Killzone 3 vs. next Bungie title vs. next Infinity Ward CoD game (Modern Warfare 3?)

In hindsight, I think Sony should've pushed R2 a little harder and waited till September to release KZ2.  February is not the ideal time frame to put out a triple A shooter.  A September release would likely have put it head to head against Halo ODST but would give it a nice 2 month jump on the juggernaut that will be MW2.  Despite the fact that Guerilla has worked on the game for 4 years, there's a lot of features missing that the extra 6 months may have allowed them to work on (Party system, matchmaking, co-op, more maps at launch, etc).

Killzone 3 could be a title for next year...
The Game Engine is already there.
Killzone 2 was released very early this year and a sequel could hit for the Christmas 2010.

There were many great answers in here :

How long until Killzone 3?

For reference purposes of course

Yes it is conceivable that KZ3 could be ready by Fall of 2010 but I would be completely shocked if they released it that early.  Here's why...

1.  There's no way Sony will release both of their flagship FPS titles at the same time.  Despite the 4 month gap between R2 and KZ2, some could argue that the games still cannibalized each other's sales.  Sony will likely follow Microsoft's strategy of alternating their shooters each fall (Gears of War, Halo 3, Gears of War 2, Halo ODST).  Even though Resistance 3 hasn't been officially announced and probably won't be til early next year, looking at Insomniac's release schedule on the PS3 all signs point to R3 coming out next fall.  

2.  As we saw with their work on KZ2, Guerilla are perfectionists.  At this time they are still working their butts off on patches and new DLC.  I don't see them ramping down support on KZ2 anytime soon to rush KZ3 for a holiday 2010 release. Even CoD W@W, which many felt was a disappointment, took 2 years to develop despite using a completed engine.

3.  Sony loves to build hype years in advance.  Look at the anticipation and wait time for KZ2, LBP, GT5, GoWIII, Heavy Rain, etc. I could see Sony releasing a huge teaser trailer at E3 2010 for KZ3 to get the hype train rolling and then follow that with a steady stream of gameplay trailers and screenshots similar to what they're doing with GoWIII right now.  Hopefully with the success of KZ2, there will be more mainstream hype for KZ3.

late 2010....

That's possible, it could be one of the ps3's last big titles before the ps4.

Hurt Killzone 2 sales... almost 2 years after its released? How?