The strength of carboxylic acids depends on the stability of the carboxylate anion formed after losing a proton.

1. Electron-Withdrawing Groups (EWG): Groups like $-NO_2$, $-Cl$, or $-CN$ stabilize the carboxylate anion through inductive (-I) and/or resonance (-R) effects, dispersing the negative charge and increasing acidity.
2. Electron-Donating Groups (EDG): Groups like alkyl groups ($-CH_3$) or methoxy groups ($-OCH_3$) destabilize the carboxylate anion by intensifying the negative charge through inductive (+I) or resonance (+R) effects, thereby decreasing acidity.

Note: The specific structures for I, II, and III were not provided in the input. However, based on the probable answer (II > III > I), compound II likely contains the strongest electron-withdrawing group or is in a position (ortho/para) that maximizes acidity, while compound I likely contains an electron-donating group or lacks stabilizing factors.