I plan to stay in dongmen or louhu area. How to get to OCT east from both areas in Shenzhen?

Dongmen is located at Luohu District.
From there, you can take subway Longgang Line at Laojie Station, get off at Cuizhu Station, Exit B2. Then take bus no.E12 to Meisha Hospital and Sightseeing Bus Line 1 to the Knight Valley (Daxiagu) Terminus Station.

Generally speaking, you need to spend 2 days to visit both valleys.
Knight Valley has many exciting amusement facilities, which Tea Stream Valley is more like a leisure park. You can choose one according to your preference.

As there will be 3 of us, maybe it will be more economical and convenient to take a taxi there? How much approximately will it cost from dongmen to oct east by taxi?

Thank you for your reply Emma...

The taxi fare would be around CNY 100.