Now, to answer your question in the sense of who has the more RIGHT of way, then I'd have to say the ambulance. Not to say that the other 2 emergency vehicles aren't as important, however, when an ambulance is on an emergency call, it is imperative that they get there as fast as they can. Let's be honest, when it comes to ambulances, seconds or minutes can decide whether someone dies or lives. On top of that, let's not forget the ambulance has to make 2 trips: one to the scene of the emergency, and then back to a hospital. The cop only travels to the scene of the crime, as the fire truck only travels to the scene of the fire.
Now, can anyone beat my answer? Because I just owned the question.