Hi I am an Australian P R has a SriLanka Passport, how can I get a visa for 3 days for HongKong?

Sithara, you should first prepare the relevant materials for tourist visa: copy of your passport, proof of your financial support, the application form affixed with a passport-style photo, transportation and accommodation arrangement and your travel plan. After, submit the application to the consulate general of PRC to apply for it.