Romans 15:10-12 — And again he saith, Rejoice, ye Gentiles, with His people.
And again, Praise the Lord, all ye Gentiles; and laud Him, all ye people.
And again, Esaias saith, There shall be a root of Jesse, and He that shall rise to reign over the Gentiles; in Him shall the Gentiles trust.
Quote in verse 10 is from Deuteronomy 32:43 — Rejoice, O ye nations, with His people: for He will avenge the blood of His servants, and will render vengeance to His adversaries, and will be merciful unto His land, and to His people. This is from the song of Moses, referring to the defeat of the Antichrist and the start of the kingdom. The blessings of the Gentiles are dependent upon the deliverence of Israel.
The quote in verse 11 is from Psalm 117:1 — Praise the LORD, all ye nations: praise Him, all ye people.
The quote in verse 12 is from Isaiah 11:1, 10 — And there shall come forth a rod out of the stem of Jesse, and a Branch shall grow out of his roots … And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and His rest shall be glorious. This prophecy specifically points to Christ.
Paul quotes from the Law, the Psalms and the Prophets.
Verse 11 should read "people", referring to Israel.