In 2 Nephi 11:3, Nephi will bring to his side two witnesses to corroborate his understanding of the prophecies of Jesus Christ and the history of salvation. He called his brother Jacob as one witness, and Isaiah as the other. Nephi said: "And now my brother Jacob also has seen him as I have seen him, wherefore I will send their words, [meaning Isaiah’s and Jacob’s words] forth as companions." Thus, we have three witnesses testifying of these things.
When did Jacob see these things? We are not told, but in 11:4, Lehi says, "And thou hast beheld in thy youth his glory." When I see the word glory, I think of the beauty of the celestial room. I love the depictions and the feelings of what that anticipates for us. For the ancient Israelites, the Holy of Holies was the place of glory. There was found the pure altar of mercy, the mercy seat with the glory, the seraphim, the burning beings who guard and protect the presence of God. It is there in the Holy of Holies that Isaiah saw God (see Isaiah 6; 2 Nephi 16).
Lehi, Nephi and Jacob have been out in the wilderness for many years. They don’t have a temple yet, but they have certainly had experiences. They long for a temple, and they’ve been blessed with visions of seeing the glory of God almost as if they were in the Holy of Holies. They will soon build a temple. It will be the first order of business as soon as Nephi can build what will be called the City of Nephi.
Book of Mormon Central, "Who Are the Witnesses of Christ in 2 Nephi? (2 Nephi 11:2–3)," KnoWhy 37 (February 19, 2016).