We recommend a three-system landscape in which each of the central clients has its own SAP System.
This consists of a development system DEV, a quality assurance system QAS and a production system PRD. The development system contains the Customizing client CUST, the quality assurance system contains the quality assurance client QTST and the production system contains the production client PROD.
Make all changes to Customizing data and Repository objects in the Customizing client. When you release the corresponding change requests, they are transported into the quality assurance client. This means that changes to cross-client data only appear in the quality assurance client after the transport. In the quality assurance client you can test whether the transports are complete, or whether any linked changes are missing and are still in unreleased change requests. If the test is successful, the change requests are transported into the production client. The production client is completely separate from the other clients as regards cross-client data.
If you need other clients with additional roles you can set them up in one of the three systems. Set up the development test client (TEST) and the prototype client (SAND) in the development system. Set up the training client (TRNG) in the quality assurance system.