Widor's pieces often showcase a refined harmonic language and beautiful melodic writing, characteristic of French Romanticism. They demand elegant phrasing and a sophisticated understanding of color.
Mastering the use of the pedal for color and achieving precise articulation in rapid passages are the main technical hurdles.
3 pièces (Op.21) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 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 3 pièces (Op.21) lasts about 10 minutes.
3 pièces (Op.21) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.