Aluminium is usually found in +3 oxidation state. In contarast, thallium exists in +1 and +3 oxidation states. This is due to :

The atoms of group 13 elements have 3 valence electrons, two in s-subshell and one in p subshell.

The +1 oxidation state becomes more stable as we move down to the group due to inert pair effect. After the removal of 1 electron from p orbital, the remaining ns² electrons behave like stable noble gases due to inert pair effect and do not take part in compound formation. The two electrons present in the s-shell are strongly attracted by the nucleus and do not participate in bonding. This inert pair effect becomes more and more prominent on moving down the group. 

Hence, the +1-oxidation state gets more stable as we move down the group.