Please list the IDs of the top 3 professors that teaches the most courses.
IDs of the professors refers to taughtBy.p_id and professor = 1; teaches the most courses refers to max(count(course_id))
SELECT T1.p_id FROM taughtBy AS T1 INNER JOIN person AS T2 ON T1.p_id = T2.p_id WHERE T2.professor = 1 GROUP BY T1.p_id ORDER BY COUNT(*) DESC LIMIT 3