Which bus to take from Kowloon area to HK Airport from between 1am and 3am?

What is ur exact location in this area? As I know, there are overnight shuttle buses to HKIA. Anyway, general suggestion for your reference:
from Kowloon Station, it is suggested to walk to Austin Station, Jordan Road to take bus no. N11, which will take you to the airport within 50min.

Thanks for the suggestion. I was thinking about going to the airport after the 31/Dec countdown at the Avenue of Stars.

Then I suggest you walk from where you are to Kowloon Hotel, Nathan Road to take shuttle bus no. N21 or N21A to HKIA.