Kelly showed some great slides of the manuscript which Bach presented to the Musgrave of Brandenburg. His talk was insightful (for someone like me, at least) and refreshingly irreverent, especially towards Bach's influences, Corelli and Vivaldi. He explained the ritornella, and how its parts were mixed and recombined with increasing sophistication as the concerto developed. In Corelli, the orchestra played the tune while the soloist played the fancy stuff. Vivaldi, in developing the concerto, gave the soloist, as well as the orchestra, a tune.
The first movement of Concerto No. 3 plays with the conventions. Instead of having an orchestral part and a solo part, Bach gave solo parts to all three trios of violins, violas and cellos. The second movement in the manuscript consists only of two chords. According to Kelly, no one knows what Bach's intention was, but Kelly suggested that Bach at the harpsichord might have improvised on that instrument an adagio that ended with those chords. The third movement is unlike the first in that it does not give solo parts to the trios; instead, its large-scale repetitions are more in the form of French dance.