Yeah currently 3 mobs but further in my post I said that would be a big problem if those mobs include EG contents (where balance matters most). BLM can be left out if SE is trying to design content where each class is left out of something (I dont that is there intention though).
There is absolutely NO PROBLEM with different jobs shining against different mobs, whether they are endgame mobs or not. I would be totally fine with a fight that you need WHMs to kill it, as long as there was also a fight where you need BLMs to kill it instead. If there weren't different mobs that different jobs were more useful on, there wouldn't be any reason to have a bunch of different jobs in the first place.