Since they are all on the same network, just give each OH instance’s persistence the same credentials and connection info for Postgres. Then they will all save their data to the same Postgres instance and I think the same DB.
Just be sure to use unique Items names across all three instances.
Both approaches will require duplicating the Items from your remote instances on your main OH instance. You’d only really need to do that if you have rules or UI stuff in your main OH that needs information from the remote OH instances.
Note there is MQTT Event Bus [4.0.0.0;4.9.9.9] if you need to go the MQTT route.