Our group contains 7 adults and stay for 3 days at Causeway Bay.

Well, according to your situation, you are advised to purchase the AE Travel Pass. Notably, one card can only be used by one person. Each card contains one or 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).
The fare is HK$250 and HK$350 respectively (including HK$50 refundable deposit). This travel pass will save some budget in a large degree.