I believe that it all depends on how much time you can spend on each. Since you LIVE in Norway, it should be easy to spend a lot of time speaking, hearing, learning and reading the language without really having to devote much of your spare time on it.
If you have already spent a great amount of time on Spanish, then going for a more practical type of studying (reading/conversating/listening) should do the trick (rather than deliberately studying grammar and vocabulary).
So, it's more like *studying* 2 languages (Norse and Japanese) and *improving* on 1 language (Spanish). If you have enough time available, you can surely do it. But it's important to keep in mind that you should spend enough time on every language in order to not start neglecting one language.
I hope this helped you ! ;)