MW2 Perks Slot 3:

Commando
Ability: Increased melee distance
Commando Pro ability: No damage from falling
How to unlock: Waste 20 enemies using the Commando perk

Steady Aim
Ability: Increase hip-fire accuracy
Steady Aim Pro ability: You can hold your breath for a longer period of time
How to unlock: Waste by hip fire 80 enemies while using the Steady Aim perk.

Scrambler
Ability: Scrambles the enemy rader in a small radius
Scrambler Pro ability: Delays enemy Claymores, thus allowing you to avoid the explosion if you are fast enough
How to unlock: Get 50 close range kills using the scrambler perk

Ninja
Ability: Invisible to heartbeat sensors
Ninja Pro ability: Move silently
How to unlock: Get 50 close range kills while using the Ninja perk.

SitRep
Ability: Reveals enemy explosives and tactical insertions
SitRep Pro Ability: Makes enemy footsteps louder.
How to unlock: Destroy 120 detected explosives or TI’s while using the SitRep perk

Last Stand
Ability: Survive a normal death.
Last Stand Pro Ability: allowed to use frag grenades or other equipment while in Last Stand
How to unlock: ?

Bonus: How to get the Tactical Nuke in Multiplayer?

In order to get it, select the Tactical Nuke as a killstreak reward and if you manage to get a 25 killstreak, you’ll receive this amazing toy! Good luck with it!