If three electrons are available to fill five empty 3d atomic orbitals, how will the electrons be distributed in the five orbitals?

Answer is: one electron in the first three orbitals, none in the fourth or fifth orbitals.  All three electrons will have parallel spins (unpaired electrons).
This is Hund's rule - orbitals of the same energy are each filled with one electron before filling any with a second, these first electrons have the same spin. 

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