The bond formed between of benzene ring and halogen is more stable because of

resonance. It has partial double bond character. So, rate of reaction towards nucleophilic substitution gets slow. This substitution is facilitated by the presence of electron withdrawing group at ortho and para position because electron density is high at this positions.
Compound (ii) and (iii) both have one electron withdrawing group but in compound (ii) electron withdrawing ( ) group is present at ortho position, so rate of reaction in compound (ii) is more than that of (iii) while (i) has no electron withdrawing group.
Hence, the correct option is .