UnknownError: error while getting last oplog entry for begin timestamp: find command not yet implemented
You cannot add a MongoDB 3.4.x member to a MongoDB 3.0.x replica set.

Replication between mixed versions is only supported for adjacent MongoDB major releases (for example, 3.0 and 3.2 or 3.2 and 3.4), and even then generally only intended to be used transiently for the purposes of a rolling upgrade.


  what does it mean this find command not yet implemented?


The find command was introduced in the MongoDB 3.2 production release series, and MongoDB 3.4 replication expects this command to be available.


  any workaround for having this 3rd server joined in the replica set?


Install MongoDB 3.0 or 3.2 for compatibility with your existing MongoDB 3.0 replica set members. If you add a new member with MongoDB 3.2, I would recommend upgrading your other replica set members to the same release version.

For more information see:


Upgrade a replica set to MongoDB 3.2
Upgrade a replica set to MongoDB 3.4