I have a feeling that the floor opens up where the 4 monitors are toward the end of the mission. 3/4 are on and one has a black screen. In the middle of them all is a square that may open similar to how the floor when the shank spawns in on the SABER strike. From this room with the 4 screens, if you keep moving forward in the mission there is a ramp you walk down with what looks like a big piece of the sleeper simulant that you can scan (the top portion of it). If I remember correctly there are two circular platforms on each side of the room (a few feet from each other actually) that maybe if stood on at the same time will activate the floor to open and the 3rd person to jump down there. I am most likely wrong but seeing how I always play solo I have not been able to test it out.