Literary: Amulek is establishing a ring of testimony around the coming of the Christ. He begins with what Alma has said. Although it is clear to all that Alma also testifies of Christ, Amulek does not make reference to that directly. This is not because it doesn’t matter, because it does. He simply need not remind them of what they have just heard of Alma.
Rather he refers to the scriptural evidence Alma gave. He specifically mentions the three prophets Alma has mentioned. It would appear that what Amulek is doing is making sure that the testimony is supported, and does so by making sure that there are at least two testators. Thus after he mentions the three prophets and their testimony, he turns from the old to the new and says “I will testify unto you of myself that these things are true.” What he has accomplished is a full set. There are two modern, and not just two but three ancient testators of this. If it is not explicit, it appears that implicitly the Nephite society accepts the Deuteronomic law of witnesses:
Deuteronomy 19:15
15 ¶ One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.
Alma and Amulek are providing a “double double” witness. They show three ancient prophets, and the two of them. Each set meets the requisite number, and in addition, the ancient and the present form another double set of witnesses. In this conception of witnesses, Alma and Amulek are presenting what they understand to be an iron-clad witness of the coming of Christ.