Connection pooling with 3 databases but 1 database as master for repmgr replication?
After research more about this topic the answer is, no, I can't, the repmgr way to work allows me to have pgbouncer connecting to master instance of postgres to handle writes and allowing to use slaves to perform reads.

I order to archive the goal exactly like I asked I must use pgbouncer for connection pooling and use pgpool2 and its Statement-Based Middleware for replication.