Nihon Sandai Jitsuroku
The sixth and last of the Rikkokushi (Six National Histories), covering the reigns of three emperors in the 9th century. A proper noun used in historical contexts.
See also: 六国史
The Nihon Sandai Jitsuroku is the historical text compiled last among the Six National Histories.
六国史 is the collective term for the six classical Japanese history texts, of which 日本三代実録 is the sixth and final one.
The title literally means 'The True Record of Three Reigns of Japan'. It is a proper name of a historical work, not a decomposable phrase.