DIY Fusion drive with 3 drives
It is possible to create a Logical Volume Group spanning over more than two disks. I doubt that it will differentiate between fast and slow HDD. The SSD part should work though.

To accomplish this put all drives in your iMac and boot to a Mavericks or Yosemite bootable installer thumb drive.

Partition all drives as simple volumes with Disk Utility.

Then open Terminal and enter diskutil list to get an overview. The result is similar to this one:

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *256.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS SSD                     255.6 GB   disk0s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1000.0 GB  disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS HDD1                    999.6 GB   disk1s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk1s3
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1000.0 GB  disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS HDD2                    999.6 GB   disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3
/dev/disk3-disk15 with sizes between 1 GB and 512 KB are part of the boot disk as well as the thumb drive.


Now create the Logical Volume Group with:

diskutil cs create Fusion /dev/disk0s2 /dev/disk1s2 /dev/disk2s2


Choose the proper disk identifiers found in the diskutil list.

The output will be similar to this one:

Started CoreStorage operation
Unmounting disk0s2
Touching partition type on disk0s2
Adding disk0s2 to Logical Volume Group
Unmounting disk1s2
Touching partition type on disk1s2
Adding disk1s2 to Logical Volume Group
Unmounting disk2s2
Touching partition type on disk2s2
Adding disk2s2 to Logical Volume Group
Creating Core Storage Logical Volume Group
Switching disk0s2 to Core Storage
Switching disk1s2 to Core Storage
Switching disk2s2 to Core Storage
Waiting for Logical Volume Group to appear
Discovered new Logical Volume Group "0AD5A644-34A7-4B44-A9BC-CD37411B8910"
Core Storage LVG UUID: 0AD5A644-34A7-4B44-A9BC-CD37411B8910


Now create a new Logical Volume with:

diskutil cs createVolume lvgUUID jhfs+ Fusion 100%


with the example above this is

diskutil cs createVolume 0AD5A644-34A7-4B44-A9BC-CD37411B8910 jhfs+ Fusion 100%


This will yield the following:

The Core Storage Logical Volume Group UUID is 0AD5A644-34A7-4B44-A9BC-CD37411B8910
Started CoreStorage operation
Waiting for Logical Volume to appear
Formatting file system for Logical Volume
Initialized /dev/rdisk16 as a 2 TB case-insensitive HFS Plus volume with a 172032k journal
Mounting disk
Core Storage LV UUID: 34345C6A-421B-4F03-A6D1-4C5A144FA610
Core Storage disk: disk16
Finished CoreStorage operation


The 2 TB is erroneous: in fact a ~2,25 TB volume will be built:

...
/dev/disk16
    #:                       TYPE NAME                    SIZE       IDENTIFIER
    0:                  Apple_HFS Fusion                 *2.3 TB     disk16
                             Logical Volume on disk0s2, disk1s2, ...
                             34345C6A-421B-4F03-A6D1-4C5A144FA610
                             Unencrypted
...


Now install a new system and enjoy your DIY SuperFusion Drive.



One fact cannot be concealed: the SuperFusion Drive is even more vulnerable to drive failure than a normal Fusion drive. The failure probability is comparable to a RAID0.

Here is a comparison for 1, 2 or 3 disks:

Survival probability (for new disks and being optimistic):

Nr of disks  1st year 2nd year 3rd year 4th year 5th year
          1       98%      90%      83%      78%      73%
          2       97%      82%      69%      61%      54%
          3       95%      74%      57%      47%      40%


Considering that at least one drive is already 5 years old the survival probability is even worse. So better get a reliable backup drive!



A deleted question invisible for some or most users asks for a speed test. A (Super)Fusion Drive is no RAID0/5/6, so r/w speed directly depends on the physical disk written to/read from. Usually at least 4 GB of the SSD are kept free. One can expect that the first ~4 GB written to the DYI SuperFusion Drive are transferred with SSD speed (150-550 MB/s) and the rest with HDD speed (20-120 MB/s). The read speed depends on the location of the files (i.e. SSD or HDD) and is similar to the write speed.