Answer
The hostess knows to bring two coffees because both professors 1 and 2 consider the option where everyone wants coffee to be viable, but professor 3 knows there is no way for everyone to want coffee; hence, she does not want it.
Work Step by Step
Professors 1, 2, 3
answers: IDK, IDK, and No, not everyone wants coffee.
The hostess brings back two coffees. If professor 1 or 2 did not want coffee, they would be able to answer the question "Does everyone want coffee?" with a firm "No." By this logic, the third professor clearly does not want coffee.
