In aromatic acids presence of electron withdrawing substituent e.g. $$ - N{O_2}$$ disperses the negative charge of the anion and stablises it and hence increases the acidity of the parent benzoic acid.

Further $$O$$- isomer will have higher acidity than corresponding $$m$$ and $$p$$ isomers. Since nitro group at $$p$$-position have more pronounced electron withdrawing than $$ - N{O_2}$$ group at $$m$$-position hence the correct order is the one given above.