The three crosses that you find throughout the city form the heart of the Amsterdam coat of arms. They are St. Andrew crosses, named after the Apostle Andrew, who was martyred on such a cross. For the Amsterdam coat of arms, the three crosses are in white, atop a red shield with a black pale.
The origin of the coat of arms unknown. The black pale could refer to the water along which the city is located. Perhaps they are derived from the weapon of the Persinian family, a family from Waterland, who once owned a lot of land in Amsterdam and the surrounding area.