I run a 3 GTX 780 Ti classified in sli with 4 monitors, if i move a monitor off the primary card to ether of the other two cards that monitor will not work in sli. As soon as i turn off sli it will show back up. If his cards are in sli the monitor has to be plugged into the top card, or closest to the CPU
Just to double check this, i unplugged all my monitors and only used 1. With all 3 cards in sli it will only display the monitor on the primary card.