For users with average ratings of 3, what kind of tip length they mostly left?
average ratings of 3 refer to user_average_stars = 3; kind of tip length they mostly left refers to tip_length where MAX(COUNT(user_id));
SELECT T2.tip_length FROM Users AS T1 INNER JOIN Tips AS T2 ON T1.user_id = T2.user_id WHERE T1.user_average_stars = 3 GROUP BY T2.tip_length ORDER BY COUNT(T2.tip_length) DESC LIMIT 1