Electrons will first enter into the set of quantum numbers n=5,l=0 or n=3,l=2

According to Aufbau's principle, electron enter in the orbitals according to their energy. Energy of an orbital is proportional to the sum (n+l). If for two orbitals (n+l) is equal then electron will be filled in the orbital having lesser value of n.Hence, here electron will be filled in the orbital with  n =3, I= 2