The Brigandine Armor is located inside the Canterbury Cathedral. Head into the Cathedral and head up onto the upper level. You’ll first want to shoot the lock on the barred door on the western side of the church. Once you’ve done that, head over to that side, use the door then make your way down the stairs to the room where you’ll find the armor chest. Shoot down the bag of rubble from above to break open the stone barricade on the floor, and then you’ll be able to reach the chest.
The Brigandine Trousers are found beneath Dover Fortress. You’ll need to head through the underground area until you reach the open area that extends onto the shoreline. You’ll then come across a breakable stone barrier on the ground that you can explode using fire pots. Hop down into the secret underground area and you’ll find the chest with the trousers inside.
The Brigandine Gauntlets are found within a building in the northern portion of Beamasfield. You’ll need to get two keys to unlock the chest. The first key is found on a table in a building in central Beamasfield, then the second is found on a table near the lumbermill area.