How can I fix or restore my 3TB Fusion Drive macOS install after Boot Camp partitioning error?
I recommend to do the following:


Create a bootable macOS Sierra installer thumb drive or if you are restricted to your iMac9,1 a bootable El Capitan installer thumb drive (same link).
Boot the iMac13,2 with the thumb drive
Open Terminal in the menubar > Utilities > Terminal
Enter diskutil list and diskutil cs list to get an overview
Destroy the LVG:

diskutil cs delete lvgUUID #lvgUUID: UUID of the Logical Volume Group

Enter exit to quit Terminal.app and open Disk Utility
Completely erase both disks (each to one volume with the names e.g. "fusion1" and "fusion2" but not "Macintosh HD").
Open Terminal again and get the device identifiers again: diskutil list
Create a new LVG and LV (here I assume the 120 GB SSD has the disk identifier disk0 and the 3 TB HDD disk1 - they might be different though!):

diskutil cs create Fusion disk0 disk1
diskutil cs createVolume lvgUUID jhfs+ "Macintosh HD" 100% #replace lvgUUID with the UUID shown in the output of the previous command

Enter exit to quit Terminal.app
After finishing the iMac9,1 backup to your TM backup drive, attach the drive to your iMac13,2 and in the Utilities windows choose Restore from Time Machine Backup > Select backup source > ...choose the drive and restore it to your Macintosh HD volume.
After the restore task has finished successfully, boot your iMac13,2 normally. Check if a RecoveryHD is present by opening Terminal and entering diskutil list.
If the RecoveryHD is missing, simply reinstall Sierra with the latest Sierra installer downloaded from the App Store  - which shouldn't affect your data but creates an additional RecoveryHD - or use the Recovery Partition Creator 4.