You can start from the trivial case: you get 7 points per quiz. So, your marks are 7, 7, 7 and the average is obviously 7.
From here, you can build as many examples as you like: we need a total score of 21 distributed among the three quizzes: for example, you can choose
4, 9, 8, since 4+9+8=21, or
6, 7, 8, since 6+7+8=21
and so on.
