The Brandenburg Concerto no. 3 is a work in three movements, first presented in 1721. The first movement (with no explicit tempo marking but usually played as allegro) can also be found in reworked form as the sinfonia of the cantata Ich liebe den Höchsten von ganzem Gemüte, BWV 174. The second movement consists of a single measure with the two chords that make up a 'Phrygian half cadence' and—although there is no direct evidence to support it—it was likely that these chords are meant to surround or follow a cadenza improvised by a harpsichord or violin player. Modern performance approaches range from simply playing the cadence with minimal ornamentation (treating it as a sort of "musical semicolon"), to inserting movements from other works, to cadenzas varying in length from under a minute to over two minutes. The score calls for three violins, three violas, three cellos, and basso continuo (including harpsichord). A standard performance lasts about 10 minutes.

Reference : https://musopen.org/music/2225/johann-sebastian-bach/brandenburg-concerto-no3-in-g-major-bwv-1048/