The other day I was discussing with a friend the daily recital of Psalm 83 after the morning prayer, when this friend told me that it was also said after the evening prayer if it was said early,* and Psalms 24, 8, 29 were read instead when praying after nightfall. I found the latter custom in R' Heidenheim's prayer book, and asked an expert rabbi about its origin. He said that these three psalms were added in order to enable saying mourner's kaddish. But I fail to see in this case the necessity of saying three psalms. (Moreover, there are some additional psalms that were omitted in later editions.) Do you know other sources discussing this custom and its reasons?
* Sources of this custom are also welcome!