Are there really 3 different skills for reading hiragana, katakana and kanji ?
Learning kanji is a difficult task and it asks for a specific skill.
I their modern form, Hiragana and katakana are simple syllabic systems that really require a few weeks to master. I don't doubt archaïc kanas were more difficult to master, but not to the point to need a specific skill for each.
Also, it is unrealistic at best to believe one with a knowledge in hiragana or katakana can understand a text only written in kanji. Yet I understand it can be intersting game-wise