The Three Forms of Unity is a collective name for the Belgic Confession, the Canons of Dort, and the Heidelberg Catechism, which reflect the doctrinal concerns of continental Calvinism and are accepted as official statements of doctrine by many of the Reformed churches.
From 1618 to 1619 the Dutch government, on behalf of the Dutch Reformed Church, called and convened the Synod of Dort. Dutch delegates, along with twenty-seven Reformed representatives from eight other countries, met at this Synod of Dort, where they collectively summarized their views in what was called the "Canons of Dort".
This same Synod then added these Canons to two other documents, both of which were in common use by the Dutch Church at the time: the Heidelberg Catechism (1563) and the Belgic Confession (1561).
In so doing, the Synod sought
The forms also provide a basis upon which ecumenical efforts can proceed based on whether a body accepts the essentials laid out in these forms.
The different documents each serve different purposes.