The decreasing order of reactivity will be as follows:-
HI > HBr > HCl
In case of reactivity the HI will be the most reactive and then HBr and least reactive will be the HCl.
This is due to the fact that the reducing character of Iodine is highest and it can easily break the H-I bond and thus can react faster.
Similarly in the HBr, Bromine is smaller than Iodine but still can react faster than Chlorine .
More easy will be the bond breaking more reactive will be the halogen hydracids.