What is the electron arrangement N3?

The nitride ion, N3­­­ˉ, has an electron arrangement of 1s2, 2s2, 2p6

The elemental nitrogen, N, has an electron arrangement of 1s2, 2s2, 2p3. Since the nitride ion, N3-, has 3 extra electons, those 3 extra electrons fill the 2p orbital giving the electron arrangement above. This stabilizes the nitride ion because it is similar to a noble gas.