If you are asking if every arc has to be three posts then no. Not even your first arc has to be three posts. Your first arc WILL be your debut but this could be a short single post thing of you taking on petty criminals. 

If you are asking if every SOLO arc is limited to three posts then yes. This is just to keep everyone moving. These three posts can be long as hell if you want them to be. Team-ups are a different story as it will be up to the players involved to keep things moving. Does that answer your question?