Doctrine and Covenants 3: 16-17. Do we assume from this reading that remnants of most of the pre-Columbian inhabitants exist today as well as the Lamanites?
Without a doubt the scripture that you reference, cited below, states that the Book of Mormon [my work] will go to the descendants of the Nephites as well as the descendants of the Lamanites. So we must conclude that those people are among us. However, we are not left to that conclusion from that scripture alone, but in 1 Ne. 13:30; 2 Ne. 3:1-3; 9:53; and Alma 45:13-14 in the Book of Mormon we read that there were Nephites who joined the Lamanites and became part of that tribe. Undoubtedly, the same was true of the other tribes mentioned in the subject scripture.
Nevertheless, my work shall go forth, for inasmuch as the knowledge of a Savior has come unto the world, through the testimony of the Jews, even so shall the knowledge of a Savior come unto my people–
And to the Nephites, and the Jacobites, and the Josephites, and the Zoramites, through the testimony of their fathers. (D&C 3:16-17)