Master --> slave03 (wal_files 4 days old, not streaming replication)
The wal_files are stored into each server.
But when the slave03 was down, the wal_files weren't being copied into it. We took too long to discover that, and now we lost some wal_files.. they've been recycled...
So my question is:
Can I just do a RE-SYNC from slave01 to slave03 of the data folder?
Or do I have to sync all the database again?
If the master is successfully shipping WALs to slave03 now, re-syncing that replica from slave01 (or slave02) should work fine. By "re-sync" I assume you mean a full filesystem copy using pg_basebackup etc.