SELECT Employee.*, T.[Количество детей], AvgSal.СредЗарплата
FROM (SELECT [Табельный номер], COUNT(*) AS [Количество детей] FROM [Дети сотрудников]
        GROUP BY [Табельный номер] HAVING Count(*)>=3) AS T,
    (SELECT AVG([Заработная плата]) AS СредЗарплата FROM [Список служащих]) AS AvgSal, [Список служащих] AS Employee
WHERE T.[Табельный номер] = Employee.[Табельный номер] AND Employee.[Заработная плата] < AvgSal.СредЗарплата