I always recommend to people to get 13 Fire stations followed by 1 hospital and 3 ambulance stations or expansions as this will get you the best paying calls. After this then you can build on police or expand the other two.
You can view what missions you can currently get here: https://www.missionchief.com/einsaetze Green highlighted missions are the ones you can get
Getting 13 fire stations straight is very difficult for me for example. In my country we have like 1 or 2 fire stations in big cities and only 1 in the others. Sometimes one city is like 20-30 km away from another. Also if you are in an alliance and your expanding too much you are going to "invade" other people regions to get so many fire stations. Instead for the police department some cities have like 7-8 stations