Answer: The woman in the middle is the thief.
Why?
Because she is the only one holding something:
She has a piece of paper on her lap with a marked outline, which looks like the shape of something that was cut out or stolen (like a missing label, tag, or evidence).
The two men have nothing in their hands and look more like they were brought in for questioning, but she is the only one with an object that could be linked to a theft.
