According to the narrator, what three things are missing from what would have been there between 1874–1890?

This information is strictly offered not by the narrator of the story, but by the author himself in the preface, which he addresses to the reader. In this preface, the author explains how the world of his novel has changed since the time in which it is set. He specifies three particular elements which gave the country its character in the "historic yesterday" of the titular Virginian. These are the buffalo, the wild antelope, and the horseman—or cowboy—himself.
Wister then explains further that the horseman or "cow-puncher," once a very romantic figure in the West, has died away. Once upon a time, these men, although thought old-fashioned even in their own era, enjoyed a "brief epoch" in which they were plentiful. At the time of writing, however, these horsemen belong to the past.