C.P.E. Bach added Fuga a 3 Soggetti to the first revised version of Die Kunst der Fuge, it is based on a frühfassung of J.S. Bach. The autograph consist of 5 pages. On page 5 C.P.E. Bach noted that this is the last fugue of his father. The piece is musically partitioned in three partitions, a theme is introduced in each partition up to three themes. Relatively few combinations are made between the themes.
The first four pages belong together in the order used in all versions. The piece doesn’t finish at page 4 (bar 227), a following page will have existed. It is unlikely that page 5 belongs after page 4. First of all the notation at the end and the beginning of the staffs doesn’t fit. On the missing page 5 there will be a fis-g instead of the notes fis-gis on the present page 5. The end of page 4 corresponds with bar 33 and 34 where fis-g are used.
Another aspect that emphasize the these that page 5 doesn’t belong after page 4 is the unusual structure of the combination of the three themes looking at the place and the tonical form and the relatively large theme-less part. This is unusal in the work of Bach. Usually first two themes are combined, followed by a combination of three themes with an octave interval in the tonical form. After the tonical form there is usually a development to the dominant. The harmonizing theme is interweaved often in an earlier stage. The parts without themes are often very structured.
Fuga a 3 Soggetti is not complete at the end but possibly also within the piece, especially at the beginning of the second and third partitions, the other themes can be added there.
Bar 1 to 6 of page 5 is a composition that has a relation to the bars after the crossed out bars at the end of the first partition. The last bars are the three themes in tonical, basic form. C.P. E. Bach included the first 6 bars of page 5 at the end of Fuga a 3 Soggetti and doesn’t add the last bars with the themes.
Possibly page 5 is a design J.S. Bach made at the end of the first partition to compose a continuation and the two other themes.
A comparison between Contrapunctus 14 and Fuga 4 of the Wohltemperierte Klavier can be made. Of cause Fuga 4 is much shorter and more combination of the three themes are made. Fuga 4 of the Wohltemerpierte Klavier has interesting similarities with Contrapunctus 14 looking to the three themes and the structure.