Every class in the multiplayer has been tweaked and enhanced, with new skills and weapons, and nearly all of them for the better. If you want to know how your favorite Killzone character has benefited (or suffered) from the changes, read on as we get classy!

Engineer: The Engineer works in very much the same way you remember, which his primary focus being on spawning turrets and fixing broken ammo dumps/weapons found within each map. His biggest change is his starting weapon. No longer sporting a shotgun, the Engineer now touts a devastating LMG for increased firepower. The LMG fits this class slightly more, since a guy who spends his time hacking and spawning really shouldn't be getting up close as he was wont to do with the shotgun. 

Although long-time Engineer players will have to adapt to a new combat style, his special abilities are still crucial and powerful, and the addition of an LMG just makes him all the more brutal. 

Infiltrator: Formerly the Saboteur in Killzone 2, the Infiltrator's main job is to dress up like the enemy and then take them out with a cheap shot. Unlike his Saboteur predecessor, the Infiltrator no longer gets shafted with a selection of weak weapons, as he has now inherited the Engineer's shotgun. This has upgraded the Infiltrator from a cool character to a potentially devastating one. The disguise ability buys players just enough time to get in close for the kill, and it seems that if you can get near enough without being spotted, a kill is guaranteed every time. 

Tactician: One of the biggest changes is the Tactician, a class that has always been an original and strategically awesome one, but was the subject of many gripes in Killzone 2. The Tactician no longer gets spawn grenades, meaning you can't just run into an enemy base and then open up a wormhole for your team. Instead, there are fixed spawn points around the map that the Tactician has to unlock. 

No only that, but the "Spot and Mark" ability has now been granted to this guy instead of the Scout, which is a far more fitting ability for a strategic character. Spot and Mark will show the locations of a number of enemies on the map, which can be a real key to victory. As if that wasn't enough, the Tactician also gets his old sentry bot back, making him quite the flexible and beefy choice. 

Marksman: The Scout class returns as the Marksman, only his rifle is considerably different this time around. It no longer has a scope, although it has a better rate of fire now. As before, the Marksman's primary ability is a cloaking device that renders him practically invisible. He feels like a harder class to use this time around, although my time with him was limited. 

[Edit: Sony has informed me that there is a traditional sniper rifle with a scope as well]

Field Medic: The Medic of Killzone 2 was already an excellent class, but in Killzone 3 he is an absolute beast. His primary power is once again the ability to recover downed allies, but in addition gets his own personal sentry bot that follows him around the map. However, it's the Medic's ability to manually recover himself once downed that makes him a class to be feared. Yep, Medics now get a chance to stand back up rather than respawn once they've taken a fall, which has turned them into the ultimate revenge killer. With the heal gun and the bot, he's also an XP-reaping monster. Truly the most improved class so far. 

And that's your early look at Killzone 3's multiplayer classes. Every class is said to be getting six abilities and six weapons to choose from, so there's still much, much more to be discovered about all these characters. In this early stage though, things are looking incredibly promising.

Killzone 3 rules the body that rocks the party. No doubt about it."

http://www.destructoid.com/pax-10-getting-classy-with-killzone-3-
s-multiplayer-183271.phtml

To recap:

Engineer- Has LMG now

Infiltrator- former Sab, now has shotty

Tactician- No spawn grenades, capturable points that the Tac has to unlock. Spot-n-mark, and airbot return for him.

Marksman- Starts with unscoped STA-14 also has traditional sniper as well

Field Medic- Sentry bot that heals, and standing up after wounded. Lotsa XP