It almost does not matter what you pick as a third Desynth. While you cannot Desynth everything, between the two you have you have access to almost all of the items that you can get via Desynthesis, with the notable exceptions of many of the ARR Relic Zodiac stuff, and many of the items you can get from CUL Desynth.

If you are a fisher, as well, I'd suggest CUL, just because of the variety of items you can get from Desynthing fish. Otherwise, just pick whatever you want. There is no real inherent advantage or disadvantage to having any of the others as your third Desynth.