Before the events of the modern era, if you could rank China, Korea and Japan in terms of how rich or advanced they tended to be, what would it be? Naturally China would be the most powerful due to its size (just like today), but what about in terms of how advanced their economies were, how sophisticated the society and cultures were, education and scholarship, the architecture and arts, etc? Who was historically more advanced?
Right now Japan and South Korea are more advanced and sophisticated than China, but would it be correct to assume that before the modern era (e.g. Meiji restoration), both Japan and Korea were backwaters compared to China?
A full answer to this, looking at the economy, art, and society of the three East Asian regions throughout recorded history, will by definition encompass several abundantly documented millenniums in a geographic region larger than Western Europe. So is there any specific time you have in mind? The answers to your questions will be wildly different depending on the period in question.
Would a comparison between Ming and Qing China and Joseon Korea - so that is, from the fifteenth to early nineteenth centuries - suffice? Unfortunately I know very little about Japanese history in general.