Among courses with difficulty of 3, how many students have intellegence level of 2?
difficulty of 3 refers to diff = 3; intelligence = 2
SELECT COUNT(T1.student_id) FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id INNER JOIN course AS T3 ON T2.course_id = T3.course_id WHERE T3.diff = 3 AND T1.intelligence = 2