Central instance is your app Server, Database instance is your database, dialogue instance comes with the central instance for single host installs and thats what talks between the database instance and central instance, In a distributed system you can decentralize and have an additional dialog instance to improve performance.
Hope that helps
The SAP is basically known as 3 tier that is why it is named as R/3 architecture
1. Presentation where user interaction
2. Central Instance so called application server where all the sap program,reports
and all repository are store in it.
3. Database instance MSQL, ORACLE , MAXDB
The application server is added based on the company load and user requirement
you can add more then one application layer that is why it is famous for scalability
when the user send request it will send to application server then it will check the free workprocess availble to listen the request and send to user
In this way there will be no load on the database and the system will run smoothly
Never ever install printers on application server it will slow down the dialog process and the user will get slow speed. rather then used separate server
Hope that helps u lot for understanding between R/3