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.