Can't speak for Korean, but I'm trying both Japanese and Chinese on Duolingo and they're... doable but far from perfect, I guess? 
The Japanese course has more lessons compared to the Chinese course + a section dedicated to learning hiragana and katakana (no kanji, unfortunately
) and even stories to improve your reading comprehension skills!
I definitely recommend checking the "tips" section before starting any lesson, though, because more often than not they will include the explanations that the exercises themselves are sorely lacking. For some reason, Duolingo REALLY likes to throw at you full sentences to translate in languages that do NOT use the Roman alphabet (this is an issue with Russian too. Little to no introduction to the language or even basic words, just... BOOM, NOW TRANSLATE "Here is your coffee and this is a pizza"
).
Speaking of beginner-friendly apps, for Chinese specifically, I found Hello Chinese to be WAY better than Duolingo, at least as a complete beginner. It's 100% free and, unlike Duolingo, it will start with simple words (with pictures) + simple sentences and easy grammar explanations that are presented as you progress, rather than in a separate section.
I found the first units of Duolingo to be MUCH easier to understand after I took a bunch of lessons on Hello Chinese
other apps I'd definitely recommend are Scripts and Drops, both by the same developer: with Scripts, you can learn how to write characters in Chinese, Japanese, Korean, Russian and other languages. With Drops, you get to learn a bunch of words with the help of pictures. The free version only allows you to use the app for 5 minutes a day and you may want to use two different accounts for Drops and Scripts, as the two apps are connected and if you use your 5 minutes in one app you won't be able to use the other... but I found them to be rather useful to learn new words 
Oh! And for Japanese I definitely recommend the apps called "Japanese" and "Memory Hint": Japanese has a bunch of flashcards for characters (hiragana, katakana and kanji), while Memory Hint actually comes with two apps (one for katakana, one for hiragana) that let you memorize the characters using pictures. Flashcards to practice also included! (I found Memory Hint to be extremely useful, I've been struggling with hiragana for over a year, managed to learn most of the characters in less than a week thanks to the app XD)