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.