Can A and C be very close countries in Europe?

I understand what you ask. It is really not important how close the countries are and where are they located. You can depart from Brussels to China and after your stay there return to Amsterdam. So A (Belgium) - B (China) - C (Netherlands). Also, you are allowed to have all types of tickets, including a single one as long as your itinerary is eligible. Just have in mind that the immediate points, before and after China have to be in two different countries.