how do I get the halo 3 fog skull?

[LOGAN AND CAM]

on some of the other answers i looked at, they say that there are three guys up on the roof and i have to shoot the third and he will drop the skull but i can only see two guys,
can somebody help me out??
my halo three user name is LOGAN AND CAM if you see mee send a friend and i can help you out with some skulls too
yah thanks but i already got the skull just a couple of minutes ago

 

[blarg honk]

To get the fog skull you have to be on the level floodgate and at the start of the level when you move pass that huge missle launcher to the right of you a flood form will appear on the top of the building to your right. Quickly shoot it. It carries the fog skull.