Which country do South Koreans hate the most between – North Korea, Japan and, China?

It really depends on who you ask. The answer will be divided between the liberals and conservatives in Korea.
For conservatives – Usually they want friendly diplomatic relations with Japan, and then hate North Korea and China. Between NK and China I’d say they generally hate NK more.
 
For liberals – They are usually very anti-Japan, and then somewhat friendly / neutral to China. A big part of liberalism in Korea is also trying to better relations with North Korea. Almost all presidents from the left tried to improve North Korean relations during their administration.