ISPConfig 3 has a different website layout, so you can not restore just /var/www, you will have to copy the files and scripts for every website individually to the new directory8s).
4) This does not make sense in my opinion as you would install packages required for ispconfig 2 which might cause ISPConfig 3 to fail.
the procedure I recommend is:
1) Make a backup of the databases and data in /var/www
2) Format the server and install ISPConfig 3
3) Create all websites and email users in ispconfig 3 again and then restore the website data and mysql / postgres databases.