In the days of Helaman, the word Lamanite was being used to refer to the wicked people who had joined with the blood descendants of Laman and his early followers. Thus the wicked who joined with the Lamanites were "no more called the Nephites, becoming wicked, and wild, and ferocious, yea, even becoming Lamanites." (Helaman 3:16.)
It is quite evident that some of the Lamanites converted by Nephi and Lehi in the prison were really apostate Nephites, for Aminadab said that they had had the gospel preached unto them by "Alma, and Amulek, and Zeezrom." (Helaman 5:41.) So far as the Book of Mormon relates, the only missionary venture engaged in by these three men together was to the apostate Zoramites who were then living in the land of Antionum. (Alma 31:3-6.)