Call of Duty Modern Warfare 3 (2023) contains 12 Plate Carrier Upgrades, also known as Armor Slots. Finding all of these collectibles unlocks the Bulletproof trophy & achievement. They look like bulletproof vests. Each of them increases your armor limit by one slot.
They are exclusively found during the 6 Open Combat Missions of the campaign. Each Open Combat Mission has 2 Armor Upgrades x 6 Missions = 12 Upgrades total. You can replay the missions via Mission Select so nothing is missable. Once collected, they will disappear. They are saved instantly and you can quit to the main menu and start the next mission right away without reaching a checkpoint. If an upgrade doesn’t spawn for you, you may have collected it already.
You can track how many you found by loading up a mission and look at the bottom left corner of the screen. By default you start out with 1 armor slot. Each upgrade you find adds 1 slot. If you found everything in a mission you will have 3 armor slots total. You can also open the map, to the left of the map it will say “Armor Slots x3” in the Gear section if you found them all. If you find armor plates (found around the map) you can then hold 
/ 
to put them in your Plate Carrier for damage protection. The more Armor Slots you unlocked the more plates you can stack to negate more damage.
Collectible Types Overview:
Video Guide
Mission 2: Precious Cargo (2 Upgrades)
Armor Upgrade #1
Inside the Harbor Tower building, on the top floor, in the same room where you must grab the Manifest objective.
Armor Upgrade #2
On the ship in the north, in the same room where you must grab the GPS tracker objective.
Mission 3: Reactor (2 Upgrades)
Armor Upgrade #3
On the rooftop of a building south of the first helicopter objective.
Armor Upgrade #4
On ground floor of a building next to the south-west helicopter objective.
Mission 7: Crash Site (2 Upgrades)
Armor Upgrade #5
Inside the building in the south of the area, in a small backroom, only accessible from the outside through a door at the corner of the building (ground floor).
Armor Upgrade #6
In the building in the north-west of the area where a sentry turret is placed, in the backroom.
Mission 9: Oligarch (2 Upgrades)
Armor Upgrade #7
Accessible from the beach, look for a supply drop with a parachute that landed at the beach.
Armor Upgrade #8
Found sitting next to the bar.
Mission 10: Highrise (2 Upgrades)
Armor Upgrade #9
On 6th floor (7th on the map but 6th by the floor numbers in the building).
Armor Upgrade #10
On the 7th floor (8th on the map but 7th by the floor numbers in the building). It’s in a locked room, you must shoot the chair behind the door to open the door.
Mission 12: Gora Dam (2 Upgrades)
Armor Upgrade #11
In a box in the north-east corner of the map, guarded by a group of enemies.
Armor Upgrade #12
On a rooftop in the south-east of the map, where a sniper enemy is stationed.
That’s all 12 Armor Slot Upgrades in COD MW3. After collecting the last one you’ll unlock the Bulletproof trophy / achievement. If you missed any, you can reload the mission and open the map, left of the map it shows how many Armor Slots you found (or look at the number of armor slots you have in bottom left of screen). This will tell you where you are missing one.
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