My Fusion Drive Partitioning Seems Wrong/Strange/Wonky
This procedure is going to get you back to a &quot;factory fresh&quot; system where your 128GB SSD and 3TB (spinning) HDD are configured as a Fusion Drive.  Basically, we're going to:

Delete all partitions off all the physical drives
Create the Fusion Drive
Install macOS

Before you begin, ensure that you have

all necessary backups.
a bootable USB installer
WiFi and a working Internet connection (if you want to use Internet Recovery)

Erase the Disks
Using a bootable USB macOS installer, boot up then go into Terminal and issue the following commands:
$ diskutil partitionDisk disk0 1 GPT HFS+ MacSSD 100%

$ diskutil partitionDisk disk1 1 GPT HFS+ MacHDD 100%

Verify that the disks are wiped clean by issuing the command
$ diskutil list

You should see two disks with a single partition each.
Create the Fusion Drive
Create the Logical Volume Group
Next create a logical volume group by &quot;combining&quot; the two drives with the following command
$ diskutil coreStorage create &lt;LogicalVolumeGroupName&gt; /dev/disk1 /dev/disk2

You can use any descriptive name for the group.  Ex:  iMacLVG
When the process has finished, it will output some status information including the logical volume group UUID like below:
Core Storage LVG UUID: XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXXX

Copy that number to your clipboard.  You will need it for the next command.
Create the Logical Volume
Issue the command
$ diskutil coreStorage createVolume lvgUUID type name size

Where...

lvgUUID = the UUID you copied from the last step
type = the format you want to you.
name = the name of the Volume.
size = how big you want to make the volume.  Valid entries are in bytes (Kilo, Mega, Giga, Tera, Peta, etc.) and percentage (%)

So, to make a volume that spans the whole logical volume in HFS+ use the following example:
 $ diskutil coreStorage createVolume  XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXXX HFS+ &quot;iMac HD&quot; 100%

Once it's done, you've got a clean volume on which to install macOS.  Issue the command to verify that you've got 3 disks:  2 physical and 1 logical.
Install macOS
Exit Terminal and complete the macOS installation.  Restore your data by using the Data Migration Tool if necessary.