As for the must-see places, wow,I just want to say Xian is a fantasitcs city with a lot of antique tourist attractions. Three days are not enough to visit them. So, I chose three attractions for you, including the Museum of Terracotta Warriors and Horses in Lintong District of Xian, the Tang's Paradise in the downtown of Xian (near the Wild Goose Pagoda) and the City Wall.
The favorable transportation is city bus (as well as tour bus). Just see the site of transportation of Xian. Here gives you the link:https://www.travelchinaguide.com/cityguides/xian/transportation/

I'm the native person in Xi'an,and I recommend you a hotel that comfortable and safe,that is Mei Lun hotel which is located oppsite to the Drum tower.second,if you will visit Xi'an,you must go to these places:the Bell Tower the Drum Tower ;Terracotta Warriors and Horses;Datang Furong Garden;Famen Temple.
Third,you could find a local tour and join it,for you will never worry about the transportation and something like that.
Hope you have a good time in xi'an~~