i search such the same setup you have or you want. How have you configured ispconfig3 or / and roundcube to solve the problem?
Lets take an example:
We have seperate hosts e.g. 1 to 3. And a seperate ispconfig3 server
So the roundcube single installation will be on ispconfig3 server too. So how to tell roundcube to choose the correct server? E.g. email@example.com
is hostet on host 2. How knows roundcube to take host 2 as login server?