The correct order of strengths of the carboxylic acids 

 

is

Acidic strength $\propto -I\text{ effect }$ As oxygen is more electron withdrawing (II) and (III) shows greater I effect than (I). Thus, (I) is least acidic. Out of
(II) and (III), (II) is more acidic than (III) as distance of O increases from  COOH group, acidic strength decreases.