Of courses with 3 credit, how many students have GPA of 3.2?
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.credit = 3 AND T1.gpa = 3.2