For the 3-credit course with the easiest difficulty, how many students get an "A" in that course?
diff refers to difficulty; diff is higher means the course is more difficult in which easiest difficulty refers to diff = 1; 3-credit course refers to credit = '3'; get an "A" refers to grade = 'A' for the course;
SELECT COUNT(T1.student_id) FROM registration AS T1 INNER JOIN course AS T2 ON T1.course_id = T2.course_id WHERE T1.grade = 'A' AND T2.credit = '3' AND T2.diff = 1