There are 4 Japanese writing systems not 3

[NightBird]

And if Romaji was used to show the pronounciation? Or something like that?

[Ronin4hire]

I deliberately didn't call it romaji because romaji is just the alphabetisation of Japanese words for the benefits of Foreigners.

Japanese use the alphabet for acronyms and in advertising.

Of course you can say 国連 instead of UN etc.. but I've seen Japanese use English acronyms quite a lot.

I suppose you could make a case for Arabic numerals being another seperate system if you want to be REALLY pedantic.

Also.. I don't buy into the idea that literature is the benchmark for deciding which is Japanese or not.

First of all my definition of what is literature is probably a bit more liberal. Second of all.. I believe the modern use of the language is how we should judge these sorts of things.