The three of them have the same amount of electrons, so we can’t determine the right one on that ground. As you might know, Pauli said that electrons are always in the position of the lowest energy. So the first situation is impossible because there is a free place available that takes less energy. Then the 3rd situation is nor possible because if one electron has a different spin than the others, there is a magnetic obstruction that be prevented if the spin changes. This means that the second situation is the right one.
