The most operationally efficient solution is to use a CloudFormation template with a custom resource to create users within the database. This approach allows the automation of user creation and ensures consistency across all three accounts by using a declarative infrastructure-as-code methodology. CloudFormation by itself does not natively manage database users within RDS, so a custom resource is necessary to achieve this functionality. This solution avoids the need for manual scripting or server management, such as deploying EC2 instances to run scripts or configuring Lambda functions with cross-account access.