They are all correct sentence, and can be replaced with each other. However, the nuances of their meanings are slightly different. 뭐, 필요한 거 있어요? (O) Well, Is there anything you need? or Well, Do you need anything (more)? 뭐, 필요한 거 없어요? (O) Well, Don't you need anything (more)? or Are you satisfied? or Is that enough?