The USB Stick appears twice in Hello Charlotte Episode 3, first in the 6th Floor laboratories, and again later on the 9th Floor.
On 6F, the first USB Stick is found near one of the deceased scientists (glowing) and is needed to interact with the module containing O91, which is necessary to progress. Since the menu cannot be opened while in this room, it's impossible to view this item in the inventory.
On 9F, the second USB Stick is locked within the drawer of a desk and requires the Key to obtain. This one allows Bennett to use the laptop on said desk to view the Map needed to solve the first challenge this floor presents.