Please list the IDs of the professors that teaches more than 3 courses.
IDs of the professors refers to taughtBy.p_id and professor = 1; teaches more than 3 courses  refers to count(course_id) > 3
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 HAVING COUNT(DISTINCT T1.course_id) > 3