Add electrons to account for the charge of the anion. For Se^{2-}, add 2 electrons to the neutral Se configuration, resulting in [Ar] 3d^{10} 4s^{2} 4p^{6}. For N^{3-}, add 3 electrons to the neutral N configuration, resulting in 1s^{2} 2s^{2} 2p^{6}.