My question is mostly theoretical, but if I have an application (for example for a private clinic) which stores their patient's health information (HIPAA), accepts payments with cards and stores payment history (PCI-DSS), and stores other information not related to any of that compliance, should I use 3 separate databases instances in that case or have 1 database instance with 3 different databases?
Suppose that AWS RDS PostgreSQL/MySQL is suitable for all of the cases. I understand that having 3 separate database instances gives some benefits like independent DB scaling, maintenance windows, version upgrades, etc, but this will significantly increase costs.
Having this, what would your suggestion, should I use 1 DB instance with 3 databases inside or better to use 3 separate independent DB instances? Unfortunately, I haven't found any statements about the real need for that separation, please point me to the link if that's really mentioned somewhere.