Answer:
Every third member of the club in her physical educations class
Explanation:
The reasoning behind this is because if Millie surveys her club members who are hosting the school play, she wouldn't be able to get an accurate average number of tickets sold because the club member have a bias in terms of purchasing the tickets as they run the club. Meanwhile the physical education class has a more random assessment because every third person in the class may or may not have an interest in the play the same way the club would.