The stability of $+1$ oxidation state among $\text{Al, Ga, In}$ and $\text{Tl}$ increases in the sequence:

Due to the inert pair effect, the stability of the lower oxidation state ($+1$ for Group 13) increases as we move down the group.Order of stability: $\text{Al}<\text{Ga}<\text{In}<\text{Tl}$.(Note: For Thallium, $+1$ is more stable than $+3$).