ECS 3rd site deployment

Does anyone is aware of the steps to deploy a 3rd geo-active site ? Is it as simple as installing/configuring the 3rd VDC, joining it to the federation and adding the new VDC to the actual replication group that contains the 2 other VDCs ? Or does it require a new replication group to be created, with all 3 vdc, and migrate buckets/data ? Just wondering. It is not clearly documented.