Follow the below steps to remove one disk from each node. Make sure you have a replication factor of 3.
1. Confirm there are no under-replicated blocks from Namenode webui. Wait if you see any under-replicated blocks.
2. Go to the configuration of the first data node. And remove the unwanted data directory. =>CM=>HDFS=>Instances=>Datanode(anyone)=>configuration=>dfs.data.dir(remove the directories which are not required)
This will remove the data directories for that particular datanode.
3. You need to refresh the cluster from the CM after saving the changes.