Hi, can I buy multiple stops for train. I want to go from Hong Kong to Xi’an Beijing and Shanghai

There is no multiple stops ticket type. You need to buy the tickets for each trip separately. By the way, there is no direct rail connection between HK and Xian, you would need a transfer to Guangzhou or Shenzhen first. There are direct trains between HK and Beijing/Shanghai. They are overnight slow rides and run every next day only