When I arrive at the airport in Hongkong at 9:55am, where do I go and what do I take to go to K3?

Actually, express bus no. K3 can not be taken at the airport and it is provided freely for those who take the airport express. So you can get an airport express ticket at Any Airport Express Customer Service Centre or Ticket Issuing Machines and get the airport express to Kowloon Station. Upon your arrival, you can easily take bus no. K3 there.
By the way, you need show your airport express ticket to take the free bus no. K3, so keep your ticket well.