What ticket/ card do i need to travel in Hong Kong for 3 days? and from HKIA to Hotel Dorsett?

You can purchase the Airport Express Travel Pass. This card contains one/two AE single journey(s) plus 3 consecutive days of unlimited travel on MTR (except East Rail Line First Class, Lo Wu and Lok Ma Chau stations), Light Rail and MTR Bus (Northwest New Territories).

About the transportation, do you refer to Dorsett Hotel Mongkok? If yes, you can take the AE to Kowloon Station first. Then take the free shuttle bus line K5 to your hotel directly. :)