I graduated in Japanese and Korean at SOAS and I wouldn't have been able to take Korean units in my first year, but as I had previously studied Japanese I went into the second year. I can't comment on Chinese but in Japanese there is a compulsory J2 module which is language based and worth 2 units, leaving two remaining units, which I chose as two other Korean language modules. I went to Japan for the third year, and there is no compulsory study abroad in Korea. In the fourth year, I took two Japanese and two Korean units, but there isn't anything stopping you doing all of one or the other. Personally I did my dissertation and advanced practical Japanese on the Japanese side and intermediate Korean and 20th century Korea in the fourth year. it takes some discipline keeping up the Korean though, as a lot can be forgotten on tvhe year abroad in Japan, or in China too as I would imagine.
I found the Japanese workload higher and more strict than Korean, which, while it was also high in workload, was much more lenient in terms of marking and general level of difficulty, particularly in the language courses. The history modules are also not too bad, as the actual work that goes towards the final grade were only two essays and a final exam, at least when I was there anyway.