Which of the following statements are correct?

Two Cl atoms give a larger -I effect than one, so the first is correct. In $\mathrm{CH}{}_3\mathrm{CHClCOOH}$ Cl is on the alpha carbon, and in $\mathrm{ClCH}{}_2\mathrm{CH}{}_2\mathrm{COOH}$ on the beta carbon, so the second is wrong. Acetic acid has a +I methyl group and is weaker than formic acid. The $-\mathrm{CN}$ group is -I, so the last is correct.