I think he wants 3 servers for load balancing/ contingency. Or the query is rather if it is a good strategy.
@adamtang How many users will have email, wordpress account and how busy is existing service/server? Are you planning VPS or physical server. Each to their own and it depends on preference. If your service is super busy with heavy usage, yes - I would use multiple servers. If service is small and for simplicity - 1 server will be sufficient and a 2nd to store backups/contingency. For advance users DNS overlap etc so if anything happens to primary, you can resume service on 2nd server. I would try to make it as simple as possible.
You can install virtualmin everywhere and also make 1 to manage the others but it will require some maintenance. If you have some stats of current use, we might be able to provide better feedback.