Lecture 3 will explain the construction of a generating series for 0-cycles on S. The main conjecture is that (i) this series coincides with the q-expansion of the central derivative of a Siegel Eisenstein series of genus 2 and weight 3/2, and (ii) the restriction of this function to the `diagonal' SL(2)xSL(2) coincides with the height pairing of two generating functions for curves on S. Recent progress on this conjecture will be discussed.