Berlin got three airports because each one served different parts of the city back when it was divided between the victors of World War II. After the disintegration of the East German communist government reunified Germany in the 1990s, the city needed one central airport. The Berliners want their city to have the masses of flights like London, United Kingdom has with Heathrow International Airport, and Paris, France has with Charles De Gaulle International Airport (which is actually in Roissy, France).
But Frankfurt is the undisputed financial capital of Germany, and so it has Frankfurt International Airport, which is Germany's busiest airport. Munich is not too far behind with its Franz Josef Strauß International Airport. Despite the fact that Berlin does not have the economic prosperity Frankfurt and Munich have, it hopes to establish itself in third place.