The relative stability of +1 oxidation state of group 13 elements follows the order

The stability order of +3 and +1 oxidation states of group 13 elements will be :B+3 > Al+3 > Ga+3 > In+3 >> Tl+3 (order of +3 oxidation state)B+<< Al+ < Ga+ < In+ < Tl+ (order of +1 oxidation state)The presence of two oxidation states in p-block elements is due to the inert pair effect. Because  of the presence of poor shielding d and f-orbitals, as we move from Ga to Tl, effective  nuclear charge of these elements increases so as to hold the valence ns2- electrons tightly. It causes difficulty to the ionisation of ns2 - electrons and it remains inert, only np1 - electrons ionises to give +1 oxidation state.