The weapon is in the room during the stage, where you issued the elimination order for the first time. Jump down and you should notice collapsed floor on the left. Climb over it to the upper floor and note the destroyed metal locker - inside, there is the M39 EMR.
MG4 is, more or less, halfway to the tunnel that you walk through towards the factory premises. You will recognize that location by going along the middle because, this is where there will appear the red blazing barrels that roll down. While in the middle of the road and facing the barrels, you need to turn right to notice three stone pipes or tunnels next to each other. They are stuck into the ground and in the middle one, there is the MG4 among rubble.
The last weapon, CBJ-MS, is on the metal ramp in the factory's premises that you walked into through the tunnel. Climb up the stairs to the platform on the right (its balustrade is yellow) and run along the ramp while looking to the left. Halfway along the ramp, you should notice a rusty ramp bouncing off the platform. Jump onto that ramp and walk slowly ahead, while looking down. After a while, you should notice the weapon lying on it.