Malbim: They are the 3,000 chosen above (24:2). This is why there is a Sheva under the Beis (Radak - because it is the Kal conjugation. "U'Vachurei Yisrael Hichri'a" (Tehilim 78:31) is a Dagush (Pi'el), so the Patach under the Beis was not changed to a Sheva.)