When should I start learning a third foreign language?
My native language is Bulgarian. I'd say I'm fluent in English. I started learning German 2 years ago. I took German classes for about a year and a half besides learning it at school. I can say I understand quite a bit though I am in no way fluent and can't speak the language. Recently I've been very interested in the Norwegian language and culture. I've started a course on here and it seems like to be an easy language as it is very similar to both English and German. So my question is, should I continue with Norwegian(while also studying German) or should I reach a higher level in German first?
That's really up to you and your goals. You don't gain more hours in the day, so your progress in each language will be slower than if you were just focusing on one, but there is nothing inherently wrong with that if you really want to do two languages.
And yes, the Scandinavian languages tend to be extremely easy to understand if you are very familiar with both English and German.
I think you should start learning it. For me, German is different enough from Norwegian that I don't get them confused, but similar enough that my knowledge of Norwegian helps me learn some of the German words. Also, Norwegian is VERY easy to pronounce. I could not pronounce the German "ch" sound to save my life. :)