Satyaprakash, One half of your question is almost correct while the other half is factually incorrect. Let us analyse it as follows:-

1) Electrophilic substitution:- Now, the NO2 group is electron withdrawing grp which withdraws the electron density away from the ring through the strong -R effect and also the weaker -I effect. The effect of this electron withdrawing is observed more at the Ortho- and Para- positions as they are involved in resonance delocalisation whereas the Meta- position does not take part in resonance delocalisation and hence, the electron density is not withdrawn from it and has more electron density than that of o- and p- positions. Now, electrophile is electron deficient and it attacks an electron rich site which is meta- in this case, so the attack takes place at m- position to give meta- substituted product except in a few cases where the steric and other factors dominate. The deciding factor is the stability of the sigma- complex formed on the attack of an electrophile.

2) Nucleophilic substitution:- Similar to the above situation, the electron density is withdrawn more from the ortho- and para- positions( from the NO2 group whereas no such effect is observed at meta- position. So, electron density is lower at o- and p- positions and since nucleophile is an electron rich specie, therefore it attacks electron deficient sites which are ortho- and para- positions to give corresponding o- and p- substituted products. Therefore, in this case of nucleophilic subn, the major product is o- and p- substituted derivates.