Bikram,
If you generated a refresh token as the MCC user, then that refresh token can be used to access all of the children AdWords account that MCC has access to.
In order to access a specific account, you will need to set the
clientCustomerId (e.g., the CID for one of the 3 accounts that you have under the MCC). Every client library should have a way to set it during the run time - please refer to a client library's documentation.
Cheers,
--
Ray Tsang (AdWords API Advisor)