Welcome at the forum.
The fact that Japanese uses chinese characters will help you not to mess around too long with chinese writing. Chinese is grammatically simple so I think if you don't plan to get fluent speeking and just learn how to read chinese texts (even if it's with an dictionary) you can achieve this while learning German and Japanese. In never learned Mandarin, just started learning Japanese and learned German as my native language, so I don't really know much about the difficulties of learning these languages, but if you take your time I'm sure it's possible to leanr all three languages at the same time.
Edit: There's a typo in your other thread こんにチワワ should be こんにちわ.
If you want to express that the wa is spoken long like the y in heyyy, I think you should use something like こんにちわ～ or こんにちわぁ or something like this, but not really sure what way of writing this is the usual way in Japanese.
Reasonably fluent: English
Studying: 한국어, Fering, русский
Interested in: བོད་སྐད, संस्कृत, tiếng Việt, Suomi, العربية ,עברית and a lot more...