Support for EXTERNAL POSTGRESQL Database for Multisite HA cluster A/A/P

Hello FortiSOAR Experts.

Please suggest - whether a 3 node HA cluster, where 2 nodes are in HA Active/Active and the 3rd node is HA Passive in DR site is achievable when POSTGRESQL is externalized separately for MAIN site and DR site.

How the POSTGRESQL database replication will be achieved between MAIN and DR site, when POSTGRESQL is externalized ? 

Thanks