Select TOP 3 Categories.id, Categories.Description, avg(Products.Price) as MeanPrice, count(Products.IsAvailable) as NumberOfAvailableProducts
FROM Categories
INNER JOIN ProductCategories on  Categories.Id =
           ProductCategories.CategoryId
INNER JOIN Products on Products.Id = ProductCategories.CategoryId
WHERE Products.IsAvailable='true'
GROUP BY Categories.Id, Categories.Description
ORDER BY MeanPrice DESC