Logical Volume Group 17gb "Available Space"
I don't know what happened with your drives while installing OS X Yosemite, but I have the same Fusion drive as you but ostensibly without "glitches". 

First let's compare your and my Fusion drive, though mine is not filevaulted, but that shouldn't make a big difference.

Your SSD:

Apollo:~ arronvoce$ sudo gpt -r show /dev/diskO
     start          size  index  contents
         0             1         PMBR
         1             1         Pri GPT header
         2            32         Pri GPT table
        34             6
        48        409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
    489648     236306352      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
 236715992        262144      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
 236978136             7
 236978143            32         Sec GPT table
 236978175             1         Sec GPT header


My SSD:

machine:~ user$ sudo gpt -r show  /dev/disk0
      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    236306352      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
  236715992       262144      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
  236978136            7         
  236978143           32         Sec GPT table
  236978175            1         Sec GPT header


They have the same size and are formatted the same way.

Your HDD:

Apollo:~ arronvoce$ sudo  gpt -r show /dev/disk1
     start          size  index  contents
         0             1         PMBR
         1             1         Pri GPT header
         2            32         Pri GPT table
        34             6
        48        409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B 
    409648    5857846336      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
5858255976       1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
5859525512       1007623
5860533135            32         Sec GPT table
5860533167             1         Sec GPT header


My HDD:

machine:~ user sudo gpt -r show /dev/disk1
       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  5858853952      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
  5859263592     1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
  5860533128           7         
  5860533135          32         Sec GPT table
  5860533167           1         Sec GPT header


They have the same physical size but are formatted slightly different: after your Recovery HD (the third partition) you have some non-formatted blocks with a total size of ~516 MB (1007623 blocks)

Your Physical Volume on the SSD:

    +-&lt; Physical Volume 1212D66A-ADCC-49C2-B2BD-43180F3D1B6F
    |   ----------------------------------------------------
    |   Index:    1
    |   Disk:     disk0s2
    |   Status:   Online
    |   Size:     120988852224 B (121.0 GB)


My Physical Volume on the SSD:

    +-&lt; Physical Volume 1212D66A-ADCC-49C2-B2BD-43180F3D1B6F
    |   ----------------------------------------------------
    |   Index:    1
    |   Disk:     disk0s2
    |   Status:   Online
    |   Size:     120988852224 B (121.0 GB)


They have the same size.

Your Physical Volume on the HDD:

    +-&lt; Physical Volume 39D4DCA9-7F2E-40FA-B457-CCDD4370FF2F
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk1s2
    |   Status:   Online
    |   Size:     2999217324032 B (3.0 TB)


My Physical Volume on the HDD:

    +-&lt; Physical Volume 39D4DCA9-7F2E-40FA-B457-CCDD4370FF2F
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk1s2
    |   Status:   Online
    |   Size:     2999733223424 B (3.0 TB)


Your Physical Volume is ~516 MB (515899392 B) smaller than mine which roughly matches with your non-formatted blocks on disk1, which are "missing" now.

Your Logical Volume Group:

+-- Logical Volume Group 91F22B32-DD9C-442B-95F0-ED3DB69AC4FB
    =========================================================
    Name:         Macintosh HD
    Status:       Online
    Size:         3120206176256   B (3.1 TB)
    Free Space:   17091772416 B   (17.1 GB)


My Logical Volume Group:

+-- Logical Volume Group 91F22B32-DD9C-442B-95F0-ED3DB69AC4FB
    =========================================================
    Name:         Macintosh HD
    Status:       Online
    Size:         3120722075648 B (3.1 TB)
    Free Space:   114688 B (114.7 KB)


Your LVG is ~516 MB (515899392 B) smaller than mine, which matches with the findings above.

Your Logical Volume:

         +-&gt; Logical Volume 2E2D175C-1397-474D-900F-A9DA7B9D65D6
             ---------------------------------------------------
             Disk:                 disk2
             Status:               Online
             Size (Total):         3097259147264 B (3.1 TB)


My Logical Volume:

        +-&gt; Logical Volume 2E2D175C-1397-474D-900F-A9DA7B9D65D6
            ---------------------------------------------------
            Disk:                  disk2
            Status:                Online
            Size (Total):          3106191572992 B (3.1 TB)


Your LV is ~ 22.9 GB smaller than your LVG (while 17.1 GB are indicated).
My LV is ~ 14.5 GB smaller than my LVG (while only 115 KB are indicated!).

Your LV is ~ 8.9 GB smaller than mine, which matches again, because 22.9 - 14.5 + 0.5 (the unformatted blocks on the HDD) = 8.9 GB

Compare this with a filevaulted OS X Yosemite VM where the LVG (67.86 GB) is 365 MB bigger than the LV (67.49 GB) and the indicated free space is 12.7 MB.

Finally we have to note:


the arithmetic of diskutil is in great need of improvement
apparently some valuable hard disk space is needed/lost dealing with LVM (CoreStorage). I haven't been able to find any sources explaining this though.
You lost only 8.9 GB (which are only 2.9 ‰ of the total disk space) instead of 17.1 GB compared to my Fusion Drive.




After backing up your Mac, booting to Internet Recovery Mode or Recovery Mode and starting Terminal.app you may try to increase (or decrease) your Logical Volume with the following undocumented command after unlocking the volume with diskutil cs unlockVolume LVUUID -stdinpassphrase:

diskutil cs resizeVolume LVUUID size


Example:

diskutil cs resizeVolume 2E2D175C-1397-474D-900F-A9DA7B9D65D6 (3106191572992+x*4096)b


with x = integer and 4096=8x512 (aligned blocks).

Usually you can't expand the CoreStorage Logical Volume beyond LVG_size - ~300 MB (tested with a 64 GiB disk, I don't know how this works for bigger disks). So you may start with 3119900m. If you get a message saying "Error -69720 There is not enough free space in the Core Storage Logical Volume Group for this operation" decrease the size in 100m or 500m steps until it works. If you enter g (=Giga), m (=Mega) or k (=kilo) instead of b (=byte) diskutil will do the math and choose a proper aligned (=multiple of 4096b) value.

I have tested those steps in my aforementioned 10.10.4 VM with FileVault enabled and managed to increase the size of the volume by 6 MB to 67.5 GB. Your mileage may vary.