I'm looking forward to Killzone 3 more. I prefer the imagery in the Killzone universe, and Killzone 2's multiplayer is some of the best I've played in years. Killzone 2 trumped both Resistance games in most areas, from AI to level design. On top of that, many of the features that made the first Resistance so cool and unique were abandoned in the sequel (survival horror-esque encounters, a much grimmer tone, the weapon wheel), leading to a much weaker game in the sequel, with little improvements to make up for it.
If Insomniac returns to the grim atmosphere of Resistance: Fall of Man and puts more time into polishing the overall experience, they might be able to pull a cat out of the bag and trump Killzone 3, but I doubt it.