The school is in Wangjing, isn't? The airport express train do not stop between Sanyuanqiao and T3 (then T2). So, you should be back to Sanyuanqiao to catch the train first. There are airport bus stop around the university, such as Huajiadi Bei Li, Nanhu Qu, Wangjing Jie Xi Kou. You can get buses directly to the airport. It is faster. If you want to take a train, go 2-2.5 hours before the flight leaving.