The identity of this third branch is also unknown, but the Lord has said, Know ye not that there are more nations than one?…and I bring forth my word unto the children of men, yea, even upon all the nations of the earth? (2 Nephi 29:7).
Joseph Fielding Smith
"Now in that parable the olive tree is the House of Israel, as I have said. In its native land it began to die. So the Lord took branches like the Nephites, like the lost tribes, and like others that the Lord led off that we do not know anything about, to other parts of the earth. He planted them all over his vineyard, which is the world. No doubt he sent some of these branches into Japan, into Korea, into China. No question about it, because he sent them to all parts of the world.
"Time came when in these distant parts the trees began to decay, so the Lord sent out for the last time to gather the fruit into the harvest…
"Now there is your answer. That is the answer to these people who approach me with the question, what’s the use of going out among the Chinese, the Japanese, the Koreans, and the people of the Far East to preach the gospel to them? The answer: because they are branches of the tree, they are of the house of Israel. The Lord took the branches of the tree, grafted them into the wild olives, the Gentiles, and is bringing the Gentiles into the gospel of Jesus Christ.
“When you read that chapter through if you cannot say in your soul, ’This is absolutely a revelation from God,’ then there is something wrong with you. That tells you of history. Are we going to preach the gospel in Korea, in Japan, in China? Yes, we are. Why? Because the blood of Israel is there. And the Lord did just what he said he would do with Abraham and his posterity. He scattered them over the whole face of the earth. So now the Gentiles are sanctified by the blood of Abraham” (Answers to Gospel Questions, vol. 4, pp. 204-7 as taken from the Book of Mormon Student Manual, 1981, p. 143)