Three Forms of Unity

The Three Forms of Unity is the name that is given to the Belgic Confession, the Canons of Dort, and the Heidelberg Catechism. These three works are about the doctrinal concerns of Continental Calvinism. They are considered official statements of Church doctrine by many of the Reformed churches.
History.
From 1618 to 1619, the Dutch government, acting for the Dutch Reformed Church, called a series of meetings that would come to be called the Synod of Dort. Members from the Netherlands and twenty-seven members from eight other countries met at this Synod to discuss and come to an agreement on their views. This agreement is known as the "Canons of Dort."
This meeting then added the Canons of Dort to two other documents, the Belgic Confession and the Heidelberg Catechism. Those were both commonly used by the Dutch Church at this time. 
The reasons that they chose to do this were:
The different forms each serve different things: