This work has been composed for my highest grade students in harmony / counterpoint. The purpose was to build a piece of music for keyboard in the style of J.-S. Bach's 3 Parts Inventions (BWV 788 to 801). The idea was to give the fisrt bar only (including the whole material of the piece, this is to say the Subject and Bass line) to the students, then ask them to finish the work ! The challenge was to compose as close as possible to Bach's keyboard music. The piece I composed for this purpose �" named Sinfonia a tre �" consists in a 2 parts Invention in Fugato style: First Part (from bar 1 to 12) divided as follows: Bar 1 to 8 (letter A): from Tonic to Relative; Bar 8 to 12 (letter B): from Relative to Dominant; Second Part (12 to the end): Bar 12 to 16: from Dominant to sub-Dominant (actually bar 17, letter C); Bar 17 to the end: from sub-Dominant to the end. The fugato theme has been rythmically modified all along the piece, accompanied with 2 countersubjects (the first at bar 2, middle voice; the second from bar 8, middle voice to 9, bass). The stretto, from bar 12 (letter B) to the end is using various canonic structures (i.e. 3 parts in canon from bar 12 to 13). I give the first bars (exercice) as an extracted part.