Japanese people tend to have a longer / oval facial structure with lower cheekbones, wider / larger eyes and more pronounced noses.
Korean people tend to have flatter faces with higher / squarer cheek bones and smaller eyes with single eyelids (opposed to double).
Chinese people tend to have rounder faces than both Korean and Japanese people. China is a huge multi-ethnic country unlike Korea and Japan (which are more ethnically homogeneous) making it much harder to differentiate or generalize.