The acidic strength of the hypohalous acids follows the order

In $\mathrm{H}-\mathrm{O}-\mathrm{X}$ a more electronegative halogen pulls electron density from the O-H bond, so the proton is released more easily and the conjugate base is stabilised. Electronegativity falls Cl > Br > I, hence acidity falls $\mathrm{HOCl}>\mathrm{HOBr}>\mathrm{HOI}$.