Main Hall 3 is a Camera behind the office window. Cindy, Candy, Blank, and The Rat appear in this Camera. The camera is a vulnerable spot for Blank if he is being spotted on the camera but can also spot Shadow Candy during Night Null.
Gallery
Candy standing
Cindy inside Main Hall 3
The Rat hiding in Main Hall 3.
Blank punching the window in Main Hall 3.