Each one of the Brandenburg Concertos is different from the other. Number 3 in G major is for 3 violins, 3 violas, 3 cellos, harpsichord and double bass. The style of this concerto harks back to the concerto grosso style, that is when a small group of instruments (the concertino) within the ensemble pass musical material back and forth while the full orchestra (tutti) accompanies. Number 3 is unique in that the two groups are integrated into a whole. Bach makes but eleven instruments sound like much more because each group of three alternates between being the concertino and being part of the tutti.
The first and third movements of the concerto are written in ritornello form while the middle movement consists of a two chord cadence. Some performers play these two chords, others improvise a short cadenza, sometimes a movement from a different work of Bach's is used. Evidently there was no set rule on which route to take. Composers of the Baroque era left a lot to the performers discretion.
Brandenburg Concerto #3 by J.S. Bach :