The correct order of strengths of the carboxylic acids:

The acidic strength of carboxylic acids is increased by the presence of electron-withdrawing groups ($-I$ effect) and decreased by electron-donating groups ($+I$ effect).Chlorine is an electron-withdrawing atom.In III (${\text{CHCl}}_2\text{COOH}$), there are two $\text{Cl}$ atoms exerting a strong $-I$ effect.In II (${\text{CH}}_2\text{ClCOOH}$), there is one $\text{Cl}$ atom exerting a $-I$ effect.In I (${\text{CH}}_3\text{COOH}$), the methyl group exerts a small $+I$ effect, which destabilizes the carboxylate ion.Thus, the correct order is III > II > I.