Bach's Three-Part Inventions, also known as Sinfonias, expand on the contrapuntal principles of the Two-Part Inventions by incorporating three voices. These pieces are cornerstones of contrapuntal study.
Achieving clarity and balance in three independent melodic lines is essential.
Sinfonias is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Sinfonias lasts about 30 minutes.
Sinfonias is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.