It's likely that neither Japanese nor Korean are related to any other languages (that we can identify) on Earth.

In other words, they're as different from each other and Chinese as the three are from English (at least in principle).

That said, Chinese early adopted a large set of symbols to represent words. So a given character or symbol can mean "sun," another may mean "moon." Chinese was an early colonializing power, and when it wasn't conquering and settling it had great influence on nearby cultures. Those cultures borrowed a lot of those Chinese symbols or characters for their own use--but the grammar, the pronunciation, were completely different. This means that very often if you're good at (old style) Chinese characters you have a decent idea about what a Japanese text is about. Korean pretty much abandoned the Chinese characters in the 1800s, but you still need to know them if you want to read works printed or hand-copied back then.

Since the Chinese symbol didn't indicate pronunciation, that's okay--it's used in modern "Chinese" for a variety of different "Chinese". Many of these varieties of Chinese are mutually unintelligible--but if you know Mandarin and are a fluent reader, with a little effort you can read Cantonese reasonably well. You can't hold a conversation in the language, but they share some core grammatical principles (like French and Italian do) and a system for representing speech. It's just that the character for "sun" may have two completely different words behind it, but both words mean "sun".

Sumerian is another linguistic isolate--we don't know of a relative that it had. But Hittite--distantly related to Latin and English--as well as Assyrian, which is related to Hebrew and Arabic, borrowed a lot of the Sumerian writing system. In some cases that was pronunciation, and the words reflect the pronunciation in Hittite or Assyrian; but sometimes Sumerian was like Chinese, where a symbol indicated a word. Then Hittite and Assyrian don't show pronunciation but the "word-meaning."

The Korean vocabulary is approximately 60 percent Chinese. I'm learning mandarin now and the similarity is so great, I'm finding it much easier than Korean to learn to read. Changing the Chinese characters to hangul didn't change their meaning. The pronunciation, grammar and word order is different.

One time, my brother and I met up with her in Hong Kong. She doesn't understand a word of Cantonese, but she could read almost everything, on account of written Japanese using much of the written Chinese words. The meaning is the same, even if the spoken word is completely different. If we and a German are shown a picture of a horse, we both know what is meant, even though we see "a horse" and the German sees "ein Pferd."