There are some good quality editions of the Three Forms of Unity available in English. You might expect me to say this, but I do think that the CanRC editions are the level best. There could be some improvements here and there, but overall they represent a good level of fidelity to the original texts, readability, and the inclusion of extensive biblical references. However, from time to time, it can be not only interesting but also helpful to consult the original texts. With the advent of digitalization of old books, we are more blessed than ever with easy access to these materials.
So, for instance, you can get a 1563 Heidelberg Catechism here. Or you could find a 1562 “major” edition of the Belgic Confession here. You could get a Latin edition of the Acts of the Synod of Dort (which would include the Canons) here. If French is more to your liking, you can find it here and here. Do I need to tell you that this kind of universal access to these materials is unprecedented?
However, consulting those texts will not give the whole story. Just with the Belgic Confession, the earliest texts are from 1561 and they’re not (yet) available online. Neither are other important early editions. This is where J. N. Bakhuizen van den Brink’s De Nederlandse Belijdenisgeschriften comes in. This is a volume that includes critical texts of the Three Forms of Unity. Besides historical introductions (in Dutch), he also has extensive critical annotations for each of the confessions. This is the must-have book for serious, scholarly study of the TFU. There is another book sometimes referred to: Niemeyer’s Collectio Confessionum. Niemeyer is outdated and not nearly as comprehensive or reliable as Bakhuizen van den Brink. The good news is that Bakhuizen van den Brink is now available in its entirety online. You can find it here. On the left hand side of the page, there is a link to “Downloads,” follow that to find a complete .pdf.
Meanwhile, there is an outstanding need for a new critical text of the Three Forms of Unity that works out of English. Of course, the texts themselves would be in the original languages (French, Dutch, Latin, German), but the critical apparatus and the introductions would be in English. Perhaps someone is working on such a project, but I haven’t heard. If you’re interested in collaborating, please let me know.