I want to get  killzone 3 but i have not played 1 or 2. Is it possible to still understand the story?

3 takes place directly after 2, you could maybe get by but I would recommend playing 2.

You can totally ignore 1.

I think you kind of need the backstory of Killzone 2.

That said, you play as the merciless invaders in both games. Killzone is the only mainstream series where you play as the villains.

You should buy Killzone 2 instead, unless your main interest lies with multiplayer. However, you should not, under any circumstances, buy the original Killzone! Trust me, I recently made this mistake (despite Jeff Gerstmann's repeated warnings) and that shit is practically unplayable by today's standards.

I'd play Killzone 2 mate, it's dirt cheap these days and Killzone 3 starts immediately after the events of the previous game.

Rico is a dick.

There you go.

Killzone for the original Playstation 

Killzone 2 is in my opinion the best shooter on the PS3, you can get it for 29.99 also.

The story in the Killzone series is completely forgettable. 
 
The campaign is still fun, but you can completely ignore the characters and plot.

I wouldn't bother with the story in Killzone 1. Killzone 2 is a great game but all of the cutscenes are available on Youtube if you want to get caught up without playing the game.

@FancySoapsMan said:

The story in the Killzone series is completely forgettable. The campaign is still fun, but you can completely ignore the characters and plot.

Some would say you might have a better opinion of the series if you do ignore the plot.