I think I understand why the answer is C could be used 3 times, but I want to make sure. Is it if E is on Wednesday and there are only one other possible column (Monday) given that the other two days have a different dot notation?
Exactly, if (E) is assigned to Wednesday, the notation now says that (E) cannot go on Tuesday and Thursday--we know is the 3 dots. Since (C) could be only 1 dot, (C) could go Monday, Tuesday, and Thursday--3 times!