SELECT grade.GStudNo, ROUND( AVG(grade.Grade),2 ) AS Average, students.LName, students.FName, students.MName, students.Course
FROM students INNER JOIN grade ON students.StudNo = grade.GStudNo
WHERE GSem = '$sem' AND GYear = '$year'
GROUP BY grade.GStudNo
ORDER BY Average ASC LIMIT 3
Je l'arrondirais à deux nombres.