3 of us need to go to Kowloon. what is the cheapest and fastest metro train or bus? how much?

You can pass the border and enter Hong Kong first. There is no direct public bus.
You are advised to take East Rail Line, get off at Hung Hom Station. After that, take West Rail Line to Nam Cheong Station and Tung Chung Line to Kowloon Station, Exit B. The duration is about 65 mins. The ticket fare is 41.5 HKD per person. :)