“The Root of All Evil” is the third mission in Crysis 3 and it contains 3 blackbox locations.
Blackbox location no. 1
Jump down into the river and follow the stream. Approximately 130 meters from the secondary objective, look to the right, and you will find a dead soldier and a blackbox on the river bank.
Blackbox no. 2
Proceed down the river, until you find a yellow lifeboat (it’s approximately 240 meters from the main objective). Turn right and follow the path towards a dead end. The blackbox is located next to a dead soldier.
Blackbox no. 3
When you approach the CELL System-X Harvesting Facility, turn left, and you will pass by a crashed taxi. Keep straight down the river and you will find an entrance to a pipe tunnel. A dead soldier with a blackbox is inside the tunnel.