These are often settings of Lutheran chorales, showcasing Bach's unparalleled ability to weave complex contrapuntal textures around a given melody. Their musical distinction lies in their profound spiritual depth and intricate compositional mastery.
Mastering the independence of voices and achieving a pure, singing tone are essential technical hurdles.
3 Chorales is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
3 Chorales is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.