J.R.R. Tolkien’s The Lord of the Rings was first published in a complete three-volume edition in 1955.
The three volumes appeared as The Fellowship of the Ring in 1954, followed by The Two Towers and The Return of the King in 1955. The complete publication date is therefore often given as 1954–1955 rather than a single year. The final volume was published on 20 October 1955.
The work was issued in three volumes for practical and economic reasons, not because Tolkien originally conceived it as three unrelated novels. Tolkien generally regarded it as one large story. The volumes were printed by George Allen & Unwin in the United Kingdom.
A common error is to use 1937, the publication year of The Hobbit, or 1954, the year the first volume appeared. The question asks for the first complete three-volume edition, which was finished in 1955.