O código que você precisa é:
SELECT students.firstname, students.lastname, students.tnumber, grades.tnumber FROM students, grades WHERE grades.tnumber=students.tnumber AND (grades.Grade='A' OR grades.Grade='B' OR grades.Grade='C')
SELECT students.firstname, students.lastname, students.tnumber, grades.tnumber FROM students, grades WHERE grades.tnumber=students.tnumber AND (grades.Grade='A' OR grades.Grade='B' OR grades.Grade='C')
