Necesitas SUM
y luego ORDER BY
este valor de resumen:
SELECT TOP 3 ProductID, SUM(Quantity) as qSum
FROM Table
GROUP BY ProductID
ORDER BY qSum DESC
Necesitas SUM
y luego ORDER BY
este valor de resumen:
SELECT TOP 3 ProductID, SUM(Quantity) as qSum
FROM Table
GROUP BY ProductID
ORDER BY qSum DESC