dissociation of proton from the acid depends upon the central atoms's electronegativity
More is the electronegativity of halogen ⇒ more is its pull on the oxygen ⇒ easier will the bond breakage of O-H ⇒ higher the acidity
In terms of E.N :
Cl > Br > I
so HClO₃ is highest acidity with the highest Ka value
