Here is the full list of all 42 Killzone 3 Multiplayer trophies - there are 24 bronze, 14 silver, 3 gold and 1 platinum.
Collect all Killzone 3 Multiplayer trophies
Reach the rank of Brigade-General I
Kill a total of 1000 enemy players online
Demolish both buildings by winning Search & Destroy as ISA and Helghast in Salamun Market
While flying in mid-air, shoot down and kill another flying Jetpack player in Tharsis Depot
Kill any player that is using the Minigun pickup in Mobile factory
Kill 5 different ISA players in a single match while playing in Radec Academy
Brutal Melee 3 enemy players from behind while disguised in a single match while playing as an Infiltrator
While playing as a Marksman, kill 5 enemy Marksmen in a single match
As an Engineer, successfully hack 3 enemy turrets in a single match
Kill a total of 500 enemy players online
Get 5 kills with the Minigun pickup in a single match while playing in Mobile Factory
Deactivate the missile launch while playing in Lente Missile Base
Using Proximity Mines, kill two enemies within one minute in Blood Gracht
Get a triple kill while playing in Stahl Arms
Kill a Marksman with the Mounted Minigun in Stahl Arms
With the Field Medic Triage ++ ability active, survive bleedout 5 times in a single match
Get a single VC9 Missile Launcher kill in Radec Academy
Reach the rank of Lieutenant I
Destroy an enemy Exoskeleton with your Jetpack while in flight in The Junkyard
Kill a flying enemy Jetpack with an Exoskeleton in The Junkyard
Unlock all Ribbons in a single match while playing Operations or Warzone
Kill 3 enemies within a single activation of the Recon ability while playing as a Tactician
Get 15 kills using secondary weapons in a single match
Get a 5 kill streak in Blood Gracht
Kill 5 enemies with 5 different primary, secondary or explosive weapons in a single match
With a Jetpack equipped, fly for 60 seconds or more in Tharsis Depot
Got a mid-air kill using the Jet Pack in any multiplayer mode
Killed a player using the Exo in any multiplayer mode
Won a match in Operations, Warzone & Guerrilla Warfare modes
With the Silent Footsteps skill active, Brutal Melee an enemy player
Kill one enemy of each career type in a single match
Destroy an enemy Sentry Turret, Sentry Drone and Medi-Droid in the same match
Killed another player while cloaked
Captured a Tactical Spawn Area for the first time
Be playing at the moment your faction secures all four Tactical Spawn Areas simultaneously in Salamun Market
Joined and completed a match as part of a squad
Killed another player while disguised
Revived a friendly player for the first time
Get killed by the exhaust from the missile launch while playing in Lente Missile Base
Used the Brutal Melee move against another player
Repaired an object for the first time