A wonky plot, poor characters, lousy delivery, and a downsized multiplayer component make KZ3 a skippable entry.
The campaign:
The campaign in Killzone 3 is basically a series of action sequences strung together by cutscenes that don't deliver any quality story points and which are usually filled with supbar writing and delivery.
There's fun to be had in playing it, but you won't remember any of the plot when it's over.
The gameplay:
Pressured to expand its audience, Guerilla Games aimed to "improve" the controls and gameplay in this entry. They largely succeed, and what we have here is one of the smoothest first person shooters found on consoles. The gunplay and controls are all genre defining.
Fans of the second game may be somewhat disappointed, but I would urge them to open their mind and see if the new mechanics grow on them.
The multiplayer:
Gone are the server lists and customization options from Killzone 2. Gone is the in depth clan system of the second game as well.
There is a clan "system," but it's only a system in that it's more than a clan tag.
In addition to the downgrades from the second game, absolutely no faults from the second game were addressed.
The new upgrade system is different, but it's not better.
The main issue here is that players from all over the world are still able to choose what region they play on. This makes the matchmaking utterly worthless, as it can't ensure quality games when it's pulling from a pool of worldwide players who choose whatever server they want.
The fact that matchmaking is so pointless makes the removal of the custom servers an even greater disgrace. Frankly, if the game is going to suffer in performance online then at least let it be as enjoyable as possible.
Overall:
Killzone 3 is a good game, but it isn't much more. There are better first person shooters, one of which is the second Killzone game.
Die hard fans should plunk down money for it, but all else should avoid it, as they will likely just play a bit of the SP, a few hours of the MP, and then shelve it for good.