Write SQL query to solve given problem: What were the total orders of Medsep Group from 2018 to 2020?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT SUM(CASE WHEN SUBSTR(T1.OrderDate, -2) IN ('18', '19', '20') AND T2.`Customer Names` = 'Medsep Group' THEN 1 ELSE 0 END) FROM `Sales Orders` AS T1 INNER JOIN Customers AS T2 ON T2.CustomerID = T1._CustomerID