Why can I only have 2 partitions on my 3TB Apple Fusion Drive?
It's indeed impossible to create a third regular partition in Disk Utility.

The +-button is greyed out (please check the screenshot below):



The superior HDD is my CoreStorage Volume Group, the inferior HDD is my CoreStorage Volume (spanning over disk1s2 and disk2s2) and HDD 2 is a simple JHFS+ non-CoreStorage volume.

The complete volume list and the partition table of disk1 looks like this:

root# diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS test1                   1.5 TB     disk0s2
   3:                  Apple_HFS test2                   1.5 TB     disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *68.7 GB    disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:          Apple_CoreStorage                         33.1 GB    disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s5
   4:                  Apple_HFS HDD 2                   34.7 GB    disk1s4
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *17.2 GB    disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:          Apple_CoreStorage                         16.8 GB    disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS HDD                    *47.3 GB    disk3

root# gpt -r -vv show /dev/disk1
gpt show: /dev/disk1: mediasize=68719476736; sectorsize=512; blocks=134217728
gpt show: /dev/disk1: PMBR at sector 0
gpt show: /dev/disk1: Pri GPT at sector 1
gpt show: /dev/disk1: Sec GPT at sector 134217727
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
     409640   64562272      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
   64971912    1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
   66241448   67714096      4  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  133955544     262151         
  134217695         32         Sec GPT table
  134217727          1         Sec GPT header


After booting to another boot disk and deleting HDD 2 (disk1s4), I added two new partitions (hdd2 and hdd3) instead with gpt add...(check screenshot below)



The partition table of the same disk (the DiskIdentifier changed meanwhile from disk1 to disk0):

root$ gpt -r -vv show /dev/disk0
Password:
gpt show: /dev/disk0: mediasize=68719476736; sectorsize=512; blocks=134217728
gpt show: /dev/disk0: PMBR at sector 0
gpt show: /dev/disk0: Pri GPT at sector 1
gpt show: /dev/disk0: Sec GPT at sector 134217727
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6         
         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
     409640   64562272      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
   64971912    1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
   66241448     158552         
   66400000   30000000      4  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   96400000     200000         
   96600000   30000000      5  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  126600000    7617695         
  134217695         32         Sec GPT table
  134217727          1         Sec GPT header




Result: 

Apple is right: you can't add a third volume with Disk Utility. But it's no problem to add a second and a third (and probably even some more) regular JHFS+ volume from the command line booted from another boot disk.

The reason why Apple doesn't allow to add more volumes in Disk Utility is beyond my knowledge.