I don't want to breach the iPhone issue but to do this on 3 PCs could take so much time to backup or sync that you would lose access to the PC during the full image backup.

This is why I'm going to write "this will never happen." Even if you got what you wanted the sheer overhead of the backup and sync could leave you with no time left to actually use the machines.

All the methods I know of would have many folk write "too technical" as well.

-> Let's ask what you found that did this today, even if it cost or was subscription only.