The String Quintet No. 3 in C major, K. 515 was written by Wolfgang Amadeus Mozart. Like all of Mozart’s string quintets, it is a “viola quintet” in that it is scored for string quartet and an extra viola (two violins, two violas and cello).

The work was completed on April 19, 1787, less than a month before the completion of his stormy G Minor Quintet, K. 516. This would not be the last time that a great pair of C major/G minor works of the same form would be published in close proximity and assigned consecutive Köchel numbers. The following year, the 40th (G minor) and 41st (C major) symphonies would be completed within a few weeks of each other.

https://en.wikipedia.org/wiki/String_Quintet_No._3_(Mozart)