According to Daniel Ludlow, in the days of Helaman, the word Lamanites was being used to refer to the wicked people who had joined with the blood descendants of Laman and his early followers in fighting against the Nephites. Thus they were "no more called Nephites, becoming wicked, and wild, and ferocious, yea, even becoming Lamanites" (Helaman 3:16). [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, pp. 239-240]
Note* Thus as the story proceeds, the reader should be aware, for example, that some of the "Lamanites" converted by Nephi and Lehi in the prison were really apostate Nephites. As proof of this we find that one of these "Lamanites" named 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). [See the commentary on Helaman 5:41]