Sonin-sandai or shonin-sandai (庶人三台) is a name of gagaku (Japanese classical music). It is also written as 庶人三臺 in kanji. Sandai' seems to be derived from the name of the building of the ancient Chinese harem. It is one of the Togaku music (music from Tang [China]), and the tone of the music is Taishikicho (Taishiki tone). It is music only and is not accompanied by dancing.
It came from China and its original tone was Kotsujikicho (Kotsujiki tone). It was played at Sumahi no sechie (sumo bouts at the court), and dancing was abolished.