It happens pretty frequently. A student who I have had in class once or twice emails or stops by my office to ask for a recommendation letter for graduate school. "I've done research with Dr. So-and-so and have done an internship with Dr. Other but I need three recommendation letters for grad school. Can you help me out?" Most students find that third letter trying. At the same time we know that a recommendation letter that is simply positive without offering details or information beyond that found in the transcript is not helpful to graduate applications. So what do you do? Should you agree to write the third (likely weak) recommendation letter?