Greater the Ka value greater is the acid strength. Among these three acids HClO3 is the strongest acid, with greatest Ka and lowest pKa value (-1), Then comes HBrO3, its the second most strongest acid among the three, its Pka value is 0.7, higher than HClO3 but smaller than HIO3 (i.e. 0.77) which the weakest acid among the three.
