Can I buy a ticket on bus line 3 from airport to Xian railway station on the bus?

Oh, you can just buy the ticket from the ticket office.
In T2, the ticket office is located at Passenger Service Center (next to Exit 2).
In T3, the ticket office is located in the Arrival Hall and in the Ground Transportation Center (GTC).
You can find these places easily according to the English signs. Don't worry. :-)