Hello, we're currently running a single server installation, and plan to expand to a 3 server configuration:
Howdy @scottRstudio ! This is a fantastic question!! And unfortunately this is a bit of a complex topic to answer without a bit more context 
If you reach out to your account representative, they can help you set up an architecture call to chat through your options and some of the best practices for this type of thing!
Another resource that may be helpful - our solutions site has a bunch of information catered to different audiences, architectures, etc.
https://solutions.rstudio.com/sys-admin/architectures/rstudio-team/