Nephi begins this long section, which he quotes from the book of Isaiah, by saying that his "soul delights in the words of Isaiah" (see also how Nephi repeats these words in 2 Nephi 25:5). He says that he will liken these words unto his own people and send them forth because Isaiah "verily saw my Redeemer, even as I have seen him, and my brother, Jacob, also has seen him as I have seen him." This produces three witnesses: Isaiah, Nephi, and Jacob.
Nephi is doing this on purpose, because "by the words of three, God hath said, I will establish my word." Nephi is probably thinking of Deuteronomy 17:6 or 19:15 that requires that "at the mouth [or by the words] of three witnesses, shall the matter be established." This was a well establish point of Jewish law (see 2 Corinthians 13:1). Nephi legally qualifies them all as witnesses by establishing that each of them comes as an eyewitness. Isaiah, Jacob, and Nephi have each "seen" their Redeemer. Nephi will mention this important three-witness principle again in 2 Nephi 27:12, as will Moroni later on (see Ether 5:4; Moroni 6:7; 10:4).
Book of Mormon Central, "Can Textual Studies Help Readers Understand the Isaiah Chapters in 2 Nephi? (2 Nephi 11:8)," KnoWhy 39 (February 23, 2016).