Ok first keep in mind that the Volume Replication is like a RAID 1, so if your going to replication keep in mind there is only a Source and a Destination. See if this helps to understand better.

1. Replicate SAN 1 (Source) --> SAN2 (Destination)
2. Then replicate SAN 2 (Stop and delete the Replication then set the Destination to Source and recreate the task to point to SAN3) --> SAN3 (Set the volume to Destination and set the IP to SAN 2)
3. Finish up by replicating SAN1 --> SAN3 - Here is where SAN 1 sent the data SAN 2 so all the data on SAN 1 was sent to SAN 2 to replicate to SAN 3 as the new Destination.