The reactivity order of halogen acids with ethers is
HI > HBr> HCl
The cleavage of ethers by halogen acids involves the nucleophilic attack of the halide ion on the protonated ether molecule. Greater the nucleophilicity of the halide ion, more reactive is the halogen acid. Since the nucleophilicity of the halide ion follow the sequence I- > Br- > Cl- therefore , reactivity order of halogen acids with ethers is HI > HBr> HCl