Is it possible to have more than one replica on a PostgreSQL server?
You understood correctly. Since streaming replication keeps the replica a physical copy of the primary database cluster, you can only replicate between one primary and one replica.

There are two options:


Keep three clusters on the replica machine, one for each primary cluster.
Use logical replication.

With logical replication, you have to make sure that each subscription replicates to different tables, ideally in different databases.