>>18807160/tg/ here.
Fighter/Witch/Ninja is relatively nonexistent as either a historical archetype ideal, or as a fantasy roleplaying archetype ideal.
It would be impossible for them to base these pokemon on an idea that does not exist.
It could be possible that they didn't intend to base these pokemon on anything, and created from scratch their own archetype interaction, but that is very unlikely.
Given the opportunity to categorize any/all of these Pokemon, arguments could be made for dozens of combinations including, but not limited to:
Warrior/Oracle/Ninja
Paladin/Wizard/Rogue
Knight/Mage/Assassin
Guardian/Witch/Thief
But the fact of the matter is that all of these derivative ideas regarding what class any of these individuals are stem from the same basic archetypes of Warrior/Mage/Rogue.
It is as if to say that three pokemon are based on the idea of a jalapeno, a cake, and a pretzel.
While the pokemon may ostensibly be physically based on those items, and the characteristics they possess are shared by those objects, there is no connectivity between those objects, and if there's supposed to be a suggested interaction between them, there simply isn't any.
There is, however, a noticeable interaction between Spicy, Sweet, and Salty, and so one would, hopefully, assume that because of a suggested interaction, that the pokemon would be based on these more pure ideas that actually DO interact, rather than the more specific ideas that don't.
Though, if for some reason you believe the three starters to be completely individual and that any perceived interaction is false, then I guess it would be fine to say they were Fighter/Witch/Ninja.
But the interaction isn't false. 
