The explanation for the presence of three unpaired electrons in the nitrogen atom can be given by:

Using Hund's rule, it is possible to explain why the nitrogen atom has three unpaired electrons. This rule states that electron pairing in any orbital (s, p, d, or f) is impossible until the same sublevel's orbitals are all filled with one electron. N is configured as a $1s^{2}2s^{2}2p^3$ in electronics.
A single electron will be present in each of the three sublevels of the 2p orbital. The fourth electron must be introduced for pairing to take place.