Of course it is realistic that KFP3 could decrease, even if it does it slightly, but for now these are only predictions. We still have to see trailers but I am pretty sure Dreamworks won't be able to up the ante... again....
Dragon 1 was generic since the beginning.
Now that you mention TV, this franchise has a TV series that started right after the second movie and that could be hurting the "potential" this threequel could have. Some people think the TV series damaged HTTYD 2's grosses.