Why not peroxide effect observed in case of addition of HCl or HI with alkene but it observed with HBr only?

The peroxide effect is not noted in the case of HCI or Hl. This is due to the fact that HCI bond is too strong to be broken to produce free radicals (430.5 kJ mol-1) than the HBr bond (363.7 kJ mol-1). Hl has a lower bond energy (296.8 kJ mol-1) and in case of the addition of HI although H-I bond is easily broken by homolysis, but the iodine atoms so produced immediately react to form iodine molecules rather than attacking the double bond of carbon atom in alkene. 

           °CH3 + HI -->. CH4 +  I°

             I° + I°---> I2