It is not impossible to visit these three places in one day. No train goes to terra cotta warriors. I am living in Xian. Suggest you take airport bus to the city center and then rent a taxi for round trip to terra cotta warriors. You need one hour from the airport to the city center and one hour from the city center to Terra cotta Warriors. That is to say it is noon when you get to Teera Cotta Warriors and you can finish at 2pm or so and get back at 3pm. Then you can ask the driver to drop you off at South Gate or East Gate, where you can get to the city wall. South Gate id the most beatiful gate. In the city center, where you get off the airport bus, there are the landmark of Xian, the Bell tower and Drum Tower. You can visit there and then waiting for the airport bus back to the airport at 7pm. So, you will miss the museum. THe museum is in the south part of the city, close to Big Wild Gosse Pagoda. It is free of charge. If I remember correctly, ticket to enter (show your passport, they will give the ticket to you) is not available since 4pm. If you want to finish the three in one day, shorten the time at each place: terra cotta warriors (12:00-13:30) to Museum (15:00-16:30) to the city wall (17:00-18:00) through SOuth gate. Then have dinner to the pick-up point for the airport bus.