I was not talking about a upgarde. ISPConfig 2 and 3 are completely different software packages with a different system requirements. You can not update from ISPConfig 2 to 3.
1) Install "Perfect Debian" + ISPConfig3 on new host/primary
2) Restore /var/www + mysql backups from current ISPConfig2 server
3) What other major steps?
You can not simply restore /var/www as the directory structure and users are different. You will have to restore the website contents of every website to the new website directory on the ispconfig 3 system.
But you dont have to switch to ISPConfig 3. I just explained you above that you have selected a controlpanel that does not support multiple servers out of the box. You can run a backup system with ispconfig 2 as well, but you might have to do some manual config changes or switch the IP.