EuroAirport is one of the few airports in the world operated jointly by two countries, in this case France and Switzerland. It is governed by a 1949 international convention. The headquarters of the airport’s operations are located in Blotzheim, France. The airport is located completely on French soil; however, it has a Swiss customs area connected to Basel by a 2.5 km (1.6 mi) long customs road, thus allowing air travelers access into Switzerland bypassing French customs clearance. The airport is operated via a state treaty established in 1946 wherein the two countries (Switzerland and France) are granted access to the airport without any customs or other border restrictions. The airport’s board has 8 members each from France and Switzerland and two advisers from Germany.
This airport has three different IATA airport codes: BSL (Basel) is the Swiss code, MLH (Mulhouse) is the French code and EAP (EuroAirport) is the neutral code.
And It could possibly have four, if Germany had wanted to assign it of their own too. Normally all airports are assigned a single IATA airport code.