Between 2018 to 2020, what is the average amount of shipped orders per year under Carl Nguyen?
shipped refers to ShipDate; between 2018 and 2020 refers to SUBSTR(ShipDate, -2) IN ('18', '19', '20'); 'Carl Nguyen' is the name of Sales Team; average shipped orders per year = Divide (Count(OrderNumber), 3)
SELECT CAST(COUNT(T1.OrderNumber) AS REAL) / 3 FROM `Sales Orders` AS T1 INNER JOIN `Sales Team` AS T2 ON T2.SalesTeamID = T1._SalesTeamID WHERE (T2.`Sales Team` = 'Carl Nguyen' AND ShipDate LIKE '%/%/18') OR (T2.`Sales Team` = 'Carl Nguyen' AND ShipDate LIKE '%/%/19') OR (T2.`Sales Team` = 'Carl Nguyen' AND ShipDate LIKE '%/%/20')