Unless you will arrive at Xian early in the mornig, I think it is impossible for you to travel through these three palces in a day. It is because the terra cotta warriors and horses is not located in the downtown area of Xi'an that you must have a half day there plus time on the road.

So, this condition is that you at least arrive in Xi'an at 08:00. Firstly, take the airport shuttle Line 2 to Xi'an Railway Station (1 hour). At the railway station, take the tourism bus No.5(306)to the terra cotta warriors (40 minutes to 1 hour), visit the musuem and return to Xi'an Railway Station(3 hours). Take a taxi or city-bus to the Bell Tower or the Wild Pagoda, vice versa. Tips: the enterance to the Pagoda is close around 17:30, and the Bell Tower is close at 22:00.