I > II > III

The acidic character of hypohalous acid depends on the electronegativity of halogen. As the electronegativity value of halogen increases the acidic strength of hypohalous acid increases.

Here, Electronegativity of atom X (Cl, Br or I) attached to O decreases down the group, polarity of OH bond decreases, acid strength decreases. It means more the electronegativity of the halogen atom, more the tendency to pull electron and weaker the O - H bond and hence proton (H + ) can easily be removed.