Because there are three classes in the morning, each class has four people, and each person is on duty at most once a day, so 12 people need to be selected from 14 people to be on duty on the opening day. Obviously, there is a scheduling method of C 12 14, which is the first step;

Step 2: After the 12 people are selected, during the morning shift, 4 people are selected from the selected 12 people, including C4? 12 selection method;

The third step, in the middle class, only four people can be selected from the remaining eight people, including C4? 8 selection method;

Night shift is the remaining four people on duty, there is nothing to choose;

So the result should be the multiplication of three-step selection! Step by step, don't reorder, so there is no need to divide by A3 3.