What sales channels are used the most in the 3 places with the highest median income?
highest median income refers to Max(Median Income)
SELECT `Sales Channel` FROM ( SELECT T1.`Sales Channel` FROM `Sales Orders` AS T1 INNER JOIN `Store Locations` AS T2 ON T2.StoreID = T1._StoreID ORDER BY T2.`Median Income` DESC LIMIT 3 ) GROUP BY `Sales Channel` ORDER BY COUNT(`Sales Channel`) DESC LIMIT 1